6 of mine are straight from the wild so they have always had natural shells.. the other 3 were "beach adoptions".. they had badly painted shells and two of them moved immediately into plain shells when I brought them home.. the third one took his sweet ol time to move into a new shell.. so I immediately threw out the painted ones since they aren't the best thing for the hermits..
Mine all live in a "terrarium" of sorts.. deep layer of mixed cocoa fiber and calcium carbonate sand.. high humidity.. lots of shells to change into .. salt and fresh water plus their food. I have one who is shedding right now.. so he's buried himself down into the substrate until he's done.
When I was a kid I had several small ones.. over the years two grew to be the size of softballs (my brother killed the third).. gave them away when I moved then finally got these guys to feed my hermit addiction.. my largest now are somewhere between tennis ball and golf ball sized..
I had my original two for close to 20 years before I gave them away...