I think you answered you own question. The butt of a leader can last a long time. If it is Flourocarbon, it will last for years. Mono has a shorter life, but not a lot.
I have one butt section that I've used for several years. I just keep adding tippet and things are fine.
A leader should straighten out with just a slow, steady pull on the leader. If it doesn't, I'd get rid of it.
Ken