I had a poor crab come up with both claws and a leg missing after a bad molt.
I put him in isolation, fed him lots of mashed up food (make sure you feed PLENTY of protein and seaweeds!), and within 3 weeks he went down to molt again. I was astounded. He took 4 weeks to finish molting, and had two miniatures of his claws and his leg back!
So yes, they can molt that quickly.

make sure you have ocean salt water in that salt water dish, made from a mix meant for salt water aquariums, as there are enzymes and electrolytes you can't find in hermit crab salts.
