The Signs provided by the Council have a short duration; I think it used to have been calculated out at around 10s per level, or so, except for those of fixed duration (like 20s for Madness) or single-shot use (Hypnotism or Wracking). Thought may have had a higher duration per level, and I know that each of the attacking/defending signs has a slight variance to them (because I activate them one-two-three, and they wear off fifteen or twenty seconds apart).

The old spell, Thought Projection/118, used to have a flat 5-minute duration. This was why people would wear a crystal amulet and just rub it to destruction, so that they would have it on all the time.

OR they would use Sign of Thought once their duration got up over 5 minutes, and come out ahead.

.

One of the big ESP rewrites a dozen years or more ago jacked the duration of the crystal amulets up to about an hour, probably so people didn't have to fiddle with things so much.

<then inherently know the effect and spamming a sign or sigil a few times in town to stack to four hours (I have have had a Sunfist pc and it doesn't take long!)>

It does when your character only has a max mana of 4.

Starchitin

A severed gnomish hand crawls in on its fingertips and makes a rude gesture before quickly decaying and rotting into dust. A gust of wind quickly scatters the dust.