How long are US 500 and 1000 dollar bills valid?

The Treasury never officially invalidated any high-denomination bills even though they haven't been printed since 1945. That means they're technically still valid for making a purchase, but it wouldn't be a very good idea for at least two reasons:

First, most people have never seen one and it could cause a lot of problems for any store where you tried to spend it

Second and more importantly, nearly all high-denomination bills are worth a lot more to collectors so you could get more value by selling it instead of spending it.