How long will pyrodex keep, if put away in a dry space with no humidity?

I'm not sure, but what I can say, is that I've heard rumors that it loses its ummph year after year. That has NOT been my experience. Think about it. If a store doesn't sell all of their powder, they do not throw it away. They sell it the next year.

I've used pyrodex pellets that were 2 and a half years old from purchase date with no problems. I store it in my gunsafe.