The entire molting process is actually comprised of several stages, which includes preparing for the molt, the actual shedding of the exoskelton, the hardening of the exoskeleton and a recovery period. The shedding of the old skeleton does not take very long, but the stages that proceed and follow the shedding often take a while to complete, and these are stressful and critical stages as well.
The time it takes for a crab to complete a molt varies. As a general rule, the larger the crab, the longer the whole process will take. It is not unusual for an average sized crab to spend about four to eight weeks going through the whole process, during which time it may stay completely buried in the sand. Some crabs, however, complete the process in a significantly shorter period of time, while large crabs may take longer.
Hard as it is to not know what is going on with your crab once it has buried itself, it's best to leave a hermit crab alone and let nature take its course during molting. Check on him from time to time by looking at him but don't pick him or or try to help him with his molt. Hermit crabs have molted in a natural habitat for a long time and they know what they're doing! In addition, pulling on its body could cause significant damage and result in the loss of appendages.
Signs of Impending Molting
Your crab is digging: A healthy crab that is suddenly digging is an indicator that molting might be imminent. However, first check your thermometer and humidity gauge. If a tank is too warm, cold or dry, your crab cold be trying to dig a new habitat underneath the sand to get away from unwelcome conditions.
Your crab is eating a lot: Molting hermit crabs first store up a lot of fat and water; in fact, he may eat and drink to the likes of which you've never seen! However, all of this bingeing usually takes place at night when you're asleep so you likely won't observe it. A crab will usually store his extra fat and water in a small black "bubble" on the left side of the stomach under its fifth pair of legs.
Regenerating limbs: A crab that is missing limbs and approaching a molt will begin to regenerate them. The missing limb will look like it's growing a small, clear, gel-like nub which is actually the limb surrounded by a clear chitin sheath. As time molting approaches, the crab's gel limb will expand and become more defined.
Additional signs: Look for general lethargy, less antennae activity, antennae that appear tangled and confused, an ashy-colored exoskeleton, or eyes that look dull, like a human's cataract.