HCA: Hermit Crab Association

Hi y'all. I am a fairly new crab owner (about 2 months). I have 5 purple pinchers. The first one was a vacation give-in to one of my kids, but as soon as I got the crab home, I did some research and made it my mission to create a good crabitat for him. I got really into it, and have since "adopted" four additional crabs from a reputable seller. So, the original crab (Shelly) we got on vacation is our largest. He traded shells once since we got him in early August, and then just a few days later went underground into the sand. He's been in there for about 2 weeks. I have the tank close to me later at night and I can hear him making a TON of digging noises in there, but I can't see what is going on.

My question is, once the molting process begins, how long does it actually take? I understand they can go under for quite a while, but the amount of activity I'm hearing under the sand is pretty amazing. I wish I could film it.

Also, is he going to come out of the sand without the shell he was in? I keep reading that you'll likely find the exo next to the shell he was originally in, but I'm not sure if he'll come out of the sand with his old shell and the exo and then drop both of those and look for a bigger shell.

Thanks for reading this if you did. I'm really loving my new crabs and want them to thrive.

Welcome to crabbing!
Crabs do not leave their shells to molt. They will go underground, and sometimes move around quite a bit under there to find a 'good' spot. The actual molting process of shedding their exo does not take long. But then they stay down and eat their old exo, and wait until the new exo hardens up. You won't find their exo. They can be under anywhere for a couple of weeks to a few months. Don't worry until at least four months have gone by. I often hear mine moving around under there soon after going under - then they will be quiet for a while - then there will be some movement again shortly before they come up again.
Sometimes a crab will change shells before they go down, or shortly after they come up. But changing shells doesn't always correlate with molting.