I have bought various powders in large bulk, mil surplus, clearance sales, estate sales, etc, and sometimes those quantities last me decades. Especially if I change to a newer powder and revisit the old loads from time to time.Who doesn't shoot enough to let powder sit for years??? I have a hard time keeping most powders for over 2 years.
I have several powders on my shelves that are 30+ and 40+ years old. Especially in the pistol and shotgun powders. Many of these were bought cheap and stacked very deep.