Powder can "stay good" a long time. I have some that goes back 20 years and then some and performs just as the manuals say it should. It's been stored in varying environments but, in general, air conditioned dwellings, since I live in a part of the country that necessitates the A/C for any semblance of a civilized lifestyle All of mine has been stored in the original opened containers (kept closed, of course), except for a big batch of Unique which is in an old 1 gal. milk jug (plastic). It too, still performs to expected specs (I chrono most all my standard loads peridically). I do wonder sometimes about just how light exposure affects powder. I know I have watched various plastics deteriorate over time from outside light exposure, but I suspect this is mostly UV damage. Interesting subject....
Best Regards,
stuffit