You don't have to clean a hermit crabs tank too often. Some people do it a couple times a month, whereas others clean it once a week, and some do it way less than that. The sand should definitely be changed maybe once every couple of months, and that can get costly. You can save money on sand by just boiling the sand. It cleans the sand, so that you don't have to replace it.
Depends on how messy they are. If they throw everything around in their tank (like mine), about once a month or so is good. If they are very neat and tidy, once every five months should be okay. Make sure you clean the sponge for the water dish(es) out once a week or when necessary!