[MaxWilson]

That's generally true of all persistent effects, including Decay and poison. Seems to be 5-10 rounds or so. You'll note that if a unit flees off the edge of the map while it's poisoned several green numbers pop up at once; if it's Decaying from banefire or something it will usually disappear in a puff of blood but that's the same thing.

(Note, incidentally, that mercs always die if they rout.)

-Max