How often do hermit crabs change shells?

Hermit crabs are not born with shells, but find empty ones along beaches, etc. The reason they wear shells is for protection. Their abdomens/tails are VERY soft and moist. They also collect and store water in their shells. This also helps them breathe, as they actually have gills. They change shells as often as they feel the need to. It's best to always have a supply of extra shells for your crab to check out. I have also read that you should never try to take a crab out of its shell. It would rather let you rip it in half than give up its own shell.