That's assuming he's using urethanes for painting. Not enough info here - what type of paint? If it is urethane, then yes, 72 hours is a good minimum, though I've lettered in far less time (and let the client know that's not recommended). If it isn't a urethane but possibly an epoxy, polyurethane (not catalyzed) or enamel based paint - 1 week at a minimum.