I leave mine loaded all season which is around 18 days or so. I always check that the bullet has not moved before going into the field. Has always fired on the last day for me if I don't have any luck and have to put it back in the safe.
As others have said it will be fine. I had my muzzleloader loaded from December 21 until January 2 when I shot a deer. I hunted almost everyday in temperatures from 10-35 degrees. I left it on my workbench in the garage which was about 45 degrees between hunts. During the 11 days I had it loaded I hunted through 2 snow storms. I use electrical tape to seal the end of the barrel. There was a good conversation about this same topic a few weeks ago in the muzzleloader forum.
The only time I'll unload or shoot a muzzle loader (pull the plug for pellets or shoot for loose powder) is when I hunted a rainy day, wet snow or when the ML went from very cold to warm. I'd rather be safe than sorry and I sure was sorry one year when my ML went "frrrrrp" on a whitetail buck.
I tape the end of my ML and the projectile/sabot seals the forward end, my main concern comes from the breech which isn't sealed adding to the fact that propellants are hygroscopic (absorbs water from the air).
Edit: forgot to say that I leave my ML in outdoor temps all season when loaded. Longest I've left it loaded was well over a week with no issues but I care for it.
Good opinions above, but I'd emphasize that it depends on which propellant you're using. Using BH209, it's a safe bet you'll be good to go even a month or two later unless you get the powder actually wet. Other blackpowder substitutes are my hygroscopic (meaning that they attract and absorb humidity from the air) and need to be changed more frequently. When I'm shooting my rifle that likes 777, I change my charge every week.