Yeah, there's really no exact science or anything, but usually twice a day is fine. For the crestie you definitely want at least one of the mistings either right before lights out/sundown or even at night, since cresteds are nocturnal and like drinking water droplets from decorations or the glass sides of the tank.
I mist at least twice a day, and then sometimes I also just mist whenever I think the fake plants in my tank look a little 'dry', while making sure that I'm not doing it so much that I soak the substrate. But I live in southern California, where it's super dry.
