I have dealt with gunpowder filled artillery shell that has laid underwater for over 150 years. On removal of the fuze, the gunpowder was as good as the day it was made..
Gunpowder is not a chemical compound, it is a mechanical mixture of simple elements and chemicals and, provided the charge has not been wetted, will last indefinitely. Put simply, the main degradation mechanism is if the potassium nitrate component is allowed to dissolve and migrate away from the others. There is an argument that heat cycling and/or mechanical vibration could cause separation, but particularly with corned powder this would have to be over very extended periods..
I am well aware of the culture in hunting circles for keeping muzzle loaders loaded during the hunting season. I'm sorry, but I really think this is a selfish and dangerous attitude these days, particularly with a flintlock, which can easily self prime. The number of accidents that have occurred with loaded muzzle loaders that have left the custodianship of their original owners remains significant. Should we really be allowing this to continue..? Leaving firearms loaded is no different from planting mines in your back yard! There is no need to do it! You are not seriously suggesting that you need to rely on a muzzleloader for self protection, or you need to take that buck that has wandered on to your drive?
Muzzle loading tends to be the preserve of the "Mature" shooter, who should really know better.. none of us live forever, and do you really want to have a legacy that demonstrates that you were responsible for the death or injury of a grandchild or great grandchild after you have gone? Modern society simply does not understand guns, least of all muzzle loaders, and the potential for disaster is increasing rather than decreasing..!
Unloading a muzzle loader with one of the modern devices is quick and easy.. just do it! Never leave any firearm loaded without supervision!