Got bored. Decided to do some research on Physical Fitness and Spirit Guide!

INCOMING WALL OF TEXT!

Method:
All casts were done by the same sorcerer, with a runestaff in hand, in stance defensive, from the same room (the Broken Tower outside of Solhaven). To this extent, I minimized possible external factors as much as I possibly could. There were four trials, each with varying amounts of physical fitness and spiritual mana control. Each trial had 100 casts. There was a 4 level difference between trials 1/2 and trials 3/4

Stats and Skills of relevance:
Minor Spiritual Ranks: 44
Constitution Bonus: 24 (Dark Elf Sorcerer)
Physical Fitness: 50 (Trial 1), 75 ranks (Trial 2), and 100 ranks (Trial 3 and 4)
Spirit Mana Control: 90 ranks (Trials 1-3), 110 ranks (Trial 4)

A bit of information before the data: When you cast this spell, you are afflicted with "nausea", during which time you recieve cycles of RT, some of which can knock you prone or change your stance. You can recieve at LEAST up to 3 of these cycles, and it may be possible to recieve more, but I have not witnessed it. There are ten different levels of the nausea you can recieve, each with their own range of RT and detrimental effects. Nothing in my research shows that there is any way to influence the severity of each LEVEL of nausea; if you get the Vomit level, you will get the same range and average RT no matter how trained you are. I've divided these levels of nausea into three groups. Additionally, there is a fourth group: no nausea at all.

No Nausea
The character gets no nausea at ALL, meaning no cycles period.

Queasy
Queasy - Gives no RT or negative effects.

Moderate - Moderate nausea gives RT, but no detrimental effects otherwise.
Disoriented - Gives 1-5 RT
Light-headed (sweat) - Gives 1-9? RT
Weak-kneed - Gives 5-15 RT
Light-headed (faint) - Gives 3?-20 (needs more samples to figure this one out)
Headache/Nausea - Gives 2?-15 RT
Migraine - Gives 10-25 RT

Major - All major nausea includes some sort of detrimental stance and prone penalty.
Dizzy - Gives 5-30 RT, fully prone, forces stance to neutral.
Vertigo - Gives 15-45 RT, fully prone, forces stance to forward
Vomit - Gives 30-60 RT, kneeling, forces stance to offensive

On with the show!

Trial 1 (50 ranks of PF, 150 skill)
Cycles: 89% of casts had 2 nausea cycles, 11% had 1 nausea cycle
RT duration per cast: Mean=35.32 seconds, Range= 5-104
RT duration in cycle 1: Mean=20.82, Range=2-60
RT duration in cycle 2: Mean=16.39, Range=0-53
Casts with major nausea: 59% (14% had two major nauseas)
Cycles of major nausea: 39% (74 out of 189)
Cycle 1 major nausea: 47%
Cycle 2 major nausea: 30%
Casts with a "Queasy": 3%
Cycles with a "Queasy": 1.5% (3 out of 189, all Cycle 2)

Trial 2 (75 ranks of PF, 175 skill)
Cycles: 0% of casts had 2 nausea cycles, 100% had 1 nausea cycle (as such, all numbers are cycle 1 only)
RT duration per cast/cycle: Mean=20.14, Range=1-58
Casts with major nausea: 46%
Cycle 1 major nausea: 46%
Cycles with a "Queasy": 0%

Trial 3 (100 ranks of PF, 175 skill)
Cycles: 0% of casts had 2 nausea cycles, 82% had 1 nausea cycle, and 18% had NO nausea cycles
RT duration per cast: Mean=14.71
RT duration per cycle: Mean=17.9, Range=2-53
Casts with major nausea: 36%
Cycles with major nausea: 45% (out of 82)
Casts with a "Queasy": 0

Trial 4 (Same Physical Fitness, SMC goes from 90 to 110)
Cycles: 0% of casts had 2 nausea cycles, 79% had 1 nausea cycle, 21% had NO nausea cycles
RT duration per cast: Mean=14.83
RT duration per cycle: Mean=18.77, Range=1-57
Casts with major nausea:37%
Cycles with major nausea:47% (out of 79)

What does all this MEAN?

Average RT per cast (rounded):
50 PF/90 SMC = 35 RT
75 PF/90 SMC = 20 RT
100 PF/90 SMC = 15 RT
100 PF/110 SMC = 15 RT

Cycles per cast:
50 PF/90 SMC = 89% of two cycles, 11% for only 1
75 PF/90 SMC = 100% had only 1 cycle
100 PF/90 SMC = 82% chance of 1 cycle, 18% chance for NO cycle
100 PF/110 SMC = 79% chance of 1 cycle, 21% chance for NO cycle

What DIDN'T change?
Chance that if you got a first cycle it would be a major nausea:
47%, 46%, 45%, 47%

In other words, if you get a first cycle, its already too late for your training to help you.

Discussion:
Physical Fitness has either limited or no influence on cycle severity. My third and fourth trial do have a noticeably lower RT per cycle than the first and second trial, and at this time, I cannot account for this. Different cycles have different severity; cycle 2 is noticably less severe than cycle 1 in both overall RT and in the prevalence of major nausea. It also has an increased chance for no nausea (Queasy message). Despite major differences in total RT and odds of major nausea per cast, my cycle 1 results at 50 ranks were virtually identical to my cycle 1 results at 75 ranks (47/46% chance of major nausea, mean RT of 20.82/20.14). While the mean RT dropped a slight bit in Trials 3 and 4 (17.9 and 18.7) the prevalence of major nausea was strikingly similar (45% and 47%); if you GET a first cycle of nausea, it has about a 46% chance to be major nausea. What appears to be an initial increase to the mean RT per cycle between trials (18.73 to 20.14) is actually an illusion caused by the loss of the frequently-lower cycle 2 results, which had a mean RT of 16.39.
The frequency of the different nausea severities varied significantly between trials, but this was also primarily an effect of losing the "less severe" cycle 2. When trial 1's Cycle 1 nausea severities are compared to trial 2's Cycle 1 severities, they are quite a bit more similar. Similarly, the likelihood of receiving any one particular nausea cycle on Cycle 1 did not change in any noticeable way between all four trials.
Speaking of nausea severities: Physical Fitness had no significant or consistent influence on their range or average RT. There were minor variations in mean RT between the trials, none showed a consistent correlation and all can be attributed to random variation. In other words, if you get a Vomit nausea, no matter what your training, its going to be 30-60 seconds of RT with about a mean of 45.
Ultimately, the only true change between the casts was the prevalence of "1-cycle" casts, a shift from 11% to a full 100%, and then the appearance of "no-cycle" casts in the latter two trials. The 11% of the 1-cycles, and the ~20% chance of no nausea casts indicates to me that there is a die-roll involved, slightly influencing the time until the next cycle/overall duration of the nausea, but the randomness can eventually be trained off with enough ranks.
In the first Trial, there was no obvious correlation between the likelihood of a second cycle and the severity of the first cycle. There seems to be a separate die-roll to determine how severe each cycle is, and this has no impact on whether you will get multiple cycles. Within those 11 "1-cycle casts", I had a cycle 1 RT of 5 and a cycle 1 RT of 59, which was almost the maximum range possible. Thus, one can assume that when you cast the spell and a nausea cycle is triggered, a die-roll is made (which includes any relevant factors to the severity), and then an entirely unrelated die-roll is made to determine how soon the next nausea cycle will hit (or to slightly influence the duration of the "overall nausea"). In other words, you don't fumble the spell and then suddenly get both the worst cycles possible and more cycles. You can get multiple minor cycles, or just a single severe cycle, with little or no relation between the two.
An interesting element is the less severe nature of cycle 2. In addition to a mean RT that was 4 points lower than cycle 1 (16.39 to 20.82) and a 17% lower chance of major nausea, it also had an additional severity tier that I never saw on cycle 1, that of the "queasy" 0 RT message. It was fairly uncommon, only occurring on 3 out of 89 cycle 2 results (and never between 400 total cycle 1 results), but it may indicate that my Spiritual Mana Control is high enough to unlock the rare 0 RT for my second cycles, but not my first cycles. Further testing with a significantly higher SMC would be necessary to see if unlocking "queasies" for cycle 1 is possible. Diluting the possible pool of results with an additional possible nausea severity (from 9 to 10 possible severities) would theoretically decrease the chance of receiving major nausea.
My Spirit Mana Control trial provided virtually no information. If the website description is to be believed, it unlocks the lower tiers of nausea . . . at lower levels of SMC, it may be that nausea cycles such as Disoriented or Light (Sweat) are not possible. Since even my baseline test had 90 ranks, I may have already unlocked the majority of the benefit. I suspect it may be possible that with enough ranks, I could unlock "Queasy" for cycle 1, but I did not have enough ranks in these trials to do so. Either way, if the individual in question is 1xing at higher levels, Physical Fitness provides a far more tangible benefit. The SMC benefit may well be based on an exact rank threshold (50 ranks for Disoriented, 150 for Queasy) and may apply differently to each cycle. Further testing with both lower trained individuals AND higher trained individuals will be necessary to find further details.

Conclusion:
Further testing is needed, to an extent that it may require an Empath, Ranger, or Paladin. However, it seems that when the website documentation says that Physical Fitness influences duration of nausea, it means literally, the duration until the messaging indicates that you feel less nautious, NOT the RT of the individual cycles. Basically, it reduces the number of cycles, and ultimately, determines if there will be a cycle at all.
Exactly how much physical fitness is required to guarantee only 1 cycle (or only 2 cycles, for lower level casters), or to begin to see no nausea casts likely depends upon Constitution. Listed together in the same sentence in the spell description, Con bonus may factor into the equation as phantom ranks of Physical Fitness. This would mean more robust races could reach this threshold with less ranks, but even as a Dark Elf with a relatively low Constitution, I was able to reach 100% chance of 1 cycle with <75 ranks. Also in casual use, I witnessed a "no nausea" cast with only 94 ranks. Given that I had reached an 11% chance for a single cycle at 50 ranks, and with the knowledge that every other race in the game has a higher Constitution than this, others could expect seeing better results at 50 ranks, but perhaps not enough to completely guarantee a single-cycle.
That said, having less cycles is always beneficial, as there is nothing to stop additional cycles from giving another minute of proned RT. This is most evident in my range of RT experienced between the first three trials: a record of 104 seconds of RT version a record of 58, with a HUGE mean RT decrease from 35 seconds to 20 seconds to 15 seconds. If you want to use Spirit Guide, Physical Fitness is definitely a must. A 3-cycle cast would have a maximum RT of 180 seconds, and even if SMC reduces the POSSIBLE severity, these additional cycles are your greatest opportunity to reduce RT.

I hope you've enjoyed this excessive amount of text on Spirit Guide!