I had an arrow crab for 3 months, during which time he moulted once. It was right in the middle of the 3 months, though, so it's at least 1.5 months between moults...
However, I have heard that if there is more iodine in the water, and if they are overfed, they will moult more often. I don't know how accurate that is, though.
92-gallon corner tank, 100lbs of LR
, 140lbs of sand, 250watt 10,000K MH
, 110watts of actinic PCs, Mag 7 return, custom refugium, AquaC EV180 w/ Mag 5
Female lyretail anthias, eyelash blenny, tomato clown, saddleback clown, firefish goby, 2 sand-sifting stars