That is an impossible question to answer, due to variables in lots of areas.
The weather, humidity, can be a few conditions that can ruin a charge. The biggest problem people have with the powder is condensation. If you loaded your ML this AM and went out into the cold weather, then brought your rifle back inside where it is warm, I would push out the charge and reload.
If you loaded your ML this AM then hunted in the cold, then put it in a safe place that is not heated, you should be fine.
I always leave mine loaded between hunts, but locked in a cabinet in the unheated garage. It also has a rubber plug in the muzzle and another one over the nipple on #11 guns, or into the BP on my .25 ACP ignition.
I have only had one charge go bad on me, my first year with a muzzleloader, by bringing it into the warm house at night, then back out into the cold for a few days in a row. It went off but was quite the hang-fire, lesson learned.
Keep both ends plugged and leave it secured in the cold, and it should last for the whole season as long as it don't get wet.
Just remember to have a safe place to lock it up, especially if you have small children around. Put a "LOADED" tag on it with load information and date. Safety First! Always.