There's no consistent life expectancy to canister grade powders for reloading. This is because they control the burn rate by blending new lots with older lots of the same powder that were either faster or slower, whichever is needed to get the speed of the new lot where it needs to be. Since powder breaking down spreads like a disease throughout the whole can, acid breakdown products from the old powder can damage the new, so you can't be sure it will last any longer than the oldest powder in the blend and you don't know how old that is.
The biggest danger in these breakdown situations is the acid fumes deteriorating the deterrent coatings of the remaining good powder first, which results in increasing the powder burn rate, making it unsafe with book loads. Guns have been burst due to this phenomenon.
I can tell you the military won't keep double-base propellants more than 20 years and won't keep single-base more than 40 years, but this is bulk grade powder that has not been blended with older powder, so they know how old it is to start. With the canister grade powders, you periodically get recalls because of some of it starting to break down prematurely. Norma says, if you store their powders properly, you can count on it lasting 10 years. After that, you have to watch out for deterioration. I had an unopened container of Accurate 3100 go bad after about 20 years and some N140 that quit after about 15 years. I have other powder that is from WWII that is still fine. So there's just no getting around doing the sniff test, looking for an oily appearance or clumping of grains, and shaking a few grains onto white paper to see that no reddish-rust colored dust comes off onto the paper from it, and, in the case of metal cans or powder already in cartridge cases, checking to there is no sign of the can or case corroding.
