That's a good question. I know smokeless powder lasts a long time. I recently found a can of 2400 hiding behind a bunch of stuff and the price tag said $6.00. How old do you think that was? Well, I used it and it seemed normal. The loads didn't seem wimpy or erratic.
I would guess your Pyrodex is safe to use as long as the storage was fairly normal conditions i.e., kept indoors in climate controlled environment.