In 6 months my crabs have molted a ton. One has molted twice and the other once. How come they molt soo much. I read that they molted once every year especially because mine are at least 5 years old. My crabs are like 3-4 inches!
The smaller the crab the more often they molt. Large to Jumbo crabs can molt about a year to 18 months.
There's a sizing chart on hcru(try using the search feature) and you will be able to print it out and see exactly how big your crabs are. http://www.hermitcrabsrus.com/forum/general-questions-about-care/3021-sizing.html