When does a hermit crab change its shell?

A hermit crab changes its shell when it has grown large enough to need a bigger home. this type of crab has a soft abdomen or 'tail', which is folded up under the body, but it is not protected by a shell of its own as are most other crabs. Instead, the hermit uses empty snail shell as portable shelters, often having to fight with another crab for possession of an attractive home. Sometimes the homeseeker pulls out the original occupier, eats it and then takes over the shell.(questions and answers)