How big is the one you have presently?
They generally molt only when they have outgrown their old exoskeleton. Some just grow faster than others. Lots of factors play into this, availability of food is the biggest. The more they eat the bigger and faster they grow.
I wouldn't be concerned. As long as you can keep an eye on him. I have heard that water quality effects the molting process as well but I can't find the information to back that up. I'll "google" it later and see if i come up with anything useful.
Moby