Boarding passes usually end up in the trash on the plane. Hotel receipts go in my folder for submission with my expenses, or which I keep a copy, so I guess those I keep for a long time.

I don't think I've ever had credit not post and it's not worth the hassle, to me, to keep and track all of those.

The only one I've ever kept was my wife's Shanghai-SFO United BP that left Nov 1, crossed the date line, and landed early on Nov 1 again, the day US and UA started their FF cross-earning agreement. They had no clue in China on how to enter her DM number, so I kept the BP, knowing it would need to be submitted for 4K+ miles.