Pretty sure it’s 50 per game turn, so 6 seconds on the clock. If it’s more than that, it may be that sludge crawler has shoggoth’s absorb ability (possibly due to a mod adding it).
The code that causes regeneration also causes creatures to gain moves, which would mean that every “extra” regeneration should come with the creature getting an extra full turn for free.
Note that regenerating creatures only cause the message to appear 50% of the time.