Hey daorhgih,
I agree completely with Rcmodel. I have had some surplus powders that had certainly spent fifteen years on my shelf and had no signs of deterioration. These same surplus powders were also who knows how many years old before they were reclaimed. I have always figured if powder still looks the same and has the same odor, it is likely to be in good condition.
I keep my powder in their original containers. Lots of surplus and new powders in plastic jugs, some in metal cans, and some in cardboard containers. Even the cardboard containers are fine in my basement which does have a de-humidifier in the summer months.
Best wishes,
Dave Wile