avid,

I'm no biologist, however while helping one with a cutthroat project a few years back, he said that Sea Lice die in about three days (72 hours) in fresh water. So, if your fish still has Sea Lice on it, and they are still alive, ya know it's only been a couple of days since that fish left the salt. Look closely though, sometimes
the lice stay attached to the fish even after their dead.

Cigar