I've left both flinter and cap locks loaded for months on end...just as others have said, you shoot it once it will need cleaned, even a fouling shot. If you don't shoot it off igniting the corrosive powder, that causes rust in your barrel, your fine to keep it loaded as long as you want. If it's really really cold out, like it is now, I leave mine in the garage, truck or any other cold place, as to not draw condensation in the barrel.
a buddy of mine passed away a few years ago. We hunted together a few times, and he had 2 or 3 cap locks. Well his widow asked me if I could sell his muzzleloaders for the estate, so I said yes. I remembered he had a custom full stock Southern rifle, it was his favorite. I also remembered that he had went hunting a couple of years ago with it and had told me when he left his rifles loaded he left a orange tipped dowl rod in the barrel to remind him...the custom SMR had a dowl rod in it when I picked it up and it had been loaded since two hunting seasons ago..I took it home and capped it, it went off like a champ with no hang fire. The bore was still perfect.