I've left my rifles loaded (both flint and percussion) from mid to the end of season (unless I kilt something) until later the next spring, probably around six months with no problem. They fire just fine. I have one rifle, a flintlock Jeager, that has never miss-fired under any circumstances and has been left loaded for months at a time. I don't leave them in a garage or outside, but that may depend on where you live.
I think the bottom line is how well they are cleaned before loading. I believe, right or wrong, that miss fires result from oil or moisture being left inside the barrel from cleaning, not from the outside environment. So I go through quite a long process cleaning my guns (starting with an already clean gun) before I load them for a hunt. I clean, clean again with dry patches, clean again generously with alcohol (denatured) and again with dry patches, getting every nook and cranny of the breech, let them "air dry" for a couple of days, and then load. !