Reprofiled the bolt?Not loaded in the mag. There isnt any pressure on the plastic part of the shell just from the mag. You can leave them loaded indefinitely.
If you haven't reprofiled the bolt and you leave a fully loaded mag in the gun with the bolt closed it can deform the top shell over time from being mashed against the bottom of the bolt.
If you're going to store it like that use a slug for the top round, or store it with the bolt locked back.