I believe that the Hodgdon Powder Company started by selling WWII surplus bulk powder stored in boxcars for 25+/- years. They re-packaged it and it was about as good as the day it was made. It probably wasn't exposed to extreme heat, however.
I have a few cans of powder that I bought more than 25 years ago and it's still good, but it's been stored in cool, dry conditions.
“God save us from people who mean well.”
― Vikram Seth