I've owned two crayfish in my career as a hobbyist. I'm down to one now, but I'll explain.
Got them both at the same time. They both molted (overnight) about 2 weeks after I got them. They were about 3 inches at the time. About a day later, one attacked and pretty much consumed the other. Oh well, down to 1 now.
Since then (about 5 months), the crayfish has molted 3 additional times, all overnight. It's now up to about 7 inches, and hasn't molted in about 3 months. I found that when it had a supply of live fish to eat (darn!), he molted quite frequently. Since I've moved the cray into it's own 10G, he hasn't molted at all.