as long as the rifle is not fowled (for reason of cleaning more so then the concern for the powder)and you do not expose the rifle to extreme temperatures where moisture can effect it, you can keep loose powder as a load, as long as you could keep pellets. Actually it will keep a very long time.

We read every so often in the news of someone with a "old muzzleloader" that takes it down from the shelf, cocks it and it goes off.. Some powders have been in those old rifles for many years.

I know of one person that kept his muzzleloader loaded for almost a year. He finally brought it out to check and make sure the sights were on for the next upcoming season. Out on the range he popped a cap to clear the nipple.. Well guess who forgot to unload their rifle the season before?