Well, I guess it differs from place to place. In my case, my school has a pre-med committee, so my letters went from the writers to the pre-med directly without me seeing them and they were good for 5 years according to the pre-med office. When I applied, one of my letters was 2 years old and I was accepted with no complaints about its validity.
Anyway, check with the school you are applying to because some may have specific rules.👍