This can of RL #7 was bought in 1973 when RL #7 was the only rifle speed powder Hercules made. I just checked it and, yup, still good! I am certain of the age because I bought it.
It was stored along with all my other powders for those 41 years. Bought it to load for the 30/30 (used WW748 instead) and tried some last year in +P+ 45/70 loads last year.
This is some WW II era surplus 4831 that I bought in 1971 from a guy who had a 55 gallon drum of it. He held out for $1 a pound. I tried to wear him down more but failed. Bought 10 pounds and this is all that's left. It is just fine too. The 4831 was in paper bags for about two years.
Heat can destroy powder in a few days (160F+) but stored right and properly acid neutralized when originally made powder may have a shelf life longer than you will live. That being said there is powder from the Viet Nam era that was fertilizer when it came out of the machine.
BTW the 4831 is stored in a re-labeled WW yellow label can of 680BR that has a price tag of $3.95. I think I bought it for less than that when it was on sale.....