For me, it depends. I buy mostly 1 lb. bottles for a few reasons. I hate to pay Haz-Mat ripoff and only rarely purchase on line. I do a lot of experimenting and some powders won't get used much so any more than a pound just sits around. And my local box store just stocks 1 lb bottles. Some powders I use a lot of so I get larger containers; Universal, IMR 4964, WC 820 and BR-5 all in 8 lb jugs.
In my experience, and what I've read, smokeless powders do not have a shelf life. If properly stored they will last many decades. I had some Unique that I purchased in 1982 (?) or thereabouts, and I used the last few grain of the last bottle just a few months ago, and I saw, and measured by chrony, no difference...