How long does it take a hermit crab to pick a new shell once it leaves its old one?

Generally, a hermit crab will not leave it's old shell until it has a good option in it's claws. If your crab leaves it's shell and does not pick up a new one right away, there is likely another issue going on that you should be concerned about.

The abdomen of the crab is very delicate and soft. The abdomen also requires moisture to keep it supple and healthy, and the crab keeps water in it's shell just for this. The crab will not risk exposing it's abdomen for long unless there is a very real issue going on.

There are several reasons a crab may leave it's shell and won't pick up a new one.

One reason may be that the crab has been in a shell that has a sharp ridge or an irritant in it that has caused injury to the abdomen over time. If a crab has an injury that prevents if from picking up a new shell, it is likely he will die, as he will think that every shell he puts on has an irritant in it, when actually it is the injury to the abdomen.

Another reason may actually be that an irritant has gotten into the crab's shell, and in this case, you should remove the shell the crab has dropped, boil it for a few minutes in dechlorinated water, rinse and cool it off, then spray a little ocean salt water into the opening and place it with the hole up so he can easily slide in if he chooses. Don't be concerned if he tips the shell over and gets substrate in it, he can clean it out himself at this point.

Please note that while painted shells may be fun to look at, they can be a health hazard for your crab. With the moisture that a crab must keep in his shell, paint that may have gotten inside the shell when it was painted will become wet, then dry, causing the crab's abdomen to be permanently stuck within the shell. This always leads to death. The crabs are always forced into these shells, so there could be an irritant inside that the crab would have avoided to begin with, but he was forced to take the shell he has. Painted shells should be strongly discouraged. Do not purchase painted shells for your crabs. The paint is also possibly toxic for them. It may be non toxic for humans, but has never been tested on crabs.

Another reason he may leave his shell is because he has suffered heat damage at some point in the past few weeks. This can happen by using heat rocks (a definite no-no in a hermit crab tank) as they can heat up to over 100*F, which is a dangerous temperature for a hermit crab. It will also dry him out if he is sitting on it, also killing him. This can also happen if you are using an under tank heater without a thermostat attached to control the temperature. If you use an under tank heater, use a probe type thermometer to take the temperature of the substrate over the center of the heating mat. If it is over 82*F, you should unplug it until you can get a thermostat to maintain the temp below 82*F. A crab burrowed down over a too-hot heating pad will also "cook", and the symptoms may not be apparent for several weeks. A crab does not recover from heat damage, it is permanent and deadly.

Do not try to force your crab into a new shell. If you really must encourage him to take a new shell, place several clean shells spritzed with ocean salt water in the openings into a container and place your crab in the container with them. Then set the container into the crabitat so the temp and humidity are good for him, and just leave him alone. It may be best to cover his part of the tank so that outside movement does not startle him. he is extremely stressed, or he would not be out of his shell without picking up another to begin with.