Here's the short version: 20 years on powder is a cake walk.
Here's something interesting (the long version GRIN). You can do a couple of different tests on powder, to find the rate it's decomposing. The two tests we run are differential scanning coulometry and thermal gravitational analysis. When powder decays, it gives off heat. Think of it as leaves in the forest making not only an insulating layer on the ground, but also providing heat from their decay. Same concept.
We measure that heat. VERY VERY precisely. Doesn't have to be outside the casing for this to happen either. From the amount given off, and the rate, we can tell how old the powder is (as in if it's been stored well or not) and how much of the umph is gone.
From that, we can figure how much velocity decrease we're going to see over an extended period. It's not much, but it does add up. For 20 years, don't sweat the degredation, 'cause you're not looking at even 100 fps (in most cases) if properly stored.
There are some powders that are more stable than others. The newer "high energy" powders are made with a higher percentage nitroglycerine base, which is great if you want velocity - but not so great if you're looking for extreme long-term storage stability. By long-term, I'm talking much longer than 20 years.
Case in point: back in the 80's when I was shooting any caliber that was cheap (GRIN), I bought a bunch of Egyptian 303 Brit ammo. Cupronickel bullets, and loaded with cordite (essentially an early version of powder). I ran that though a beater Mk3, and while a touch soft, had no misfires. The headstamp said 1894 or 1896 IIRC. Still got some at home to show folks that don't believe it's possible LOL!
Is it feasible to stockpile ammo? I'd say yes, personally. But here's where the lessons were learned the hard way......
Back in '92 (I'm pretty sure it was '92 - maybe '94) there was a "primer famine". Everyone was concerned that slimy willy was gonna get his plans pushed through to institute a chemical shelf-life on primers and powder. It was chemically infeasible, but people still horded primers. I didn't shoot much at all for that year, 'cause I couldn't get primers hardly at all. The local shop rationed people to 200 primers per delivery - I'd buy my 200, my father would buy 200, and we'd send my sister in to buy 200 ;-) But sometimes it'd be two or three weeks between deliveries.
After getting caught with my pants down like that, I swore it'd never happen again. I keep between 4000 and 10,000 primers on hand of each kind. UPS delivers powder in maximum quantities of 50 pounds per package - so I get multiple shipments of powder. It's cheaper that way in any case.
I don't live too far from one of the big bullet makers. They wash the bullets when they're done with the forming process, and sometimes they dry with water spots. Those water-spotted bullets go into a "seconds" bin. I've been buying factory seconds like that from them for years. They're not hunting or self defense bullets, but sure do make good practice ammo!
For a couple of years now, I've made it a practice to walk out of chinamart with 1000 rounds of 22LR more than what I intend to use. That gets "stocked". If all I got to shoot was 22LR, it'd still take me four or five years to eat through that....... and it has the non-obvious benefit of making trips to chinamart even less frequent (it's bothersome to walk out with four things AND a K of 22LR LOL!).