I keep my 1858 replica loaded with 28 grains of APP and a 200 gr. Lee conical for a month or two at a time. All chambers are loaded and capped.
The only time I've ever had a misfire was when I tried the "candle wax over/around the nipples" trick.... It didn't work for me since the wax managed to somehow kill the primers. ( Must've wicked up into the priming compound while it was still liquid.) The loads all fired with fresh caps, however.
Anyway, I think the American Pioneer Powder would probably last a decade or two if the gun has caps that fit right and don't allow any moisture to seep in. #11 caps on a nipple that fits #10s probably won't be good for more than a few weeks, even if they're pinched to stay in place.
(I don't use any grease or lube, btw. APP doesn't need it. )
I believe the same is probably true for real black as well.
Just my observations so far. Somebody else may get entirely different results.
( I should mention that I carry this gun around with me whenever it's possible. Out around the yard, etc. It's seen some pretty foul conditions and isn't "babied". It hasn't let me down yet. )
J.C.