It just depends on their growth mostly. I have noticed that many of my new snakes shed within a week or two to me bringing them home. I think it is mostly shedding due to stress.
Offer your snake food on the regular schedule, many BPs will eat during shed, the only way to find out if yours does is to try and feed while in shed.
Its fine if they come into contact with the moss, just make sure to clean it out and rehydrate it every few days to prevent mold.