How long can the fish rest in the crust without overcooking? Would this be good served at room temperature or should it be served hot?

Hi! The crust protects the fish and keeps it moist which is great because it's harder to overcook / dry out. That said, I wouldn't go crazy and cook it an extra hour or anything. Fish doesn't need to rest the way that mammal protein does - I think it would be OK in the crust for 20-25 minutes, but I would try to time it to crack open after that. Warm is better, maybe it's just me but room temp fish sounds no bueno. Part of the fun in the pomp and circumstance of bashing the salt and revealing that pretty steaming fish!
** note, you have to hit that salt HARD to crack it - the first time I tried my meat pounder bounced right off! **