Don't know about the pellets, but I have some Pyrodex powder from when Hogdon first issued and it is just as potent as ever! I keep the lid real tight and dry.
as l ong as you keep it in a controled area. heat, humidity. light, etc. i have had a bottle of Pyrodex for 9 years and it hasnt lost its flicker. i 'd keep it in a box in a dark closet and dry and it will be good for years.
pyrocarp wont lose potency if stored properly. That triple seven stuff will go bad after its opened though. Real black powder is much preferred by everyone I know.