There are two factors determining how much value a society can derive from the body of art created within: the amount and the distribution. (ignoring "quality" because it's unaffected by the following).
The quantity created and the distribution it gets are somewhat conflicting goals: If you maximize for distribution (all works are public domain), the amount that is created may be low because artists aren't rewarded financially / can't concentrate fully on their art etc.
Currently, the compromise is to protect a work of art for 70 years after the creator's death, assuming there are very few people who'd say "oh, I'd love to write the great american novel, but I fear my great-grandchildren won't be able to collect royalties on it, so I'll go be an investment banker instead".
Arguably, 70 years is too long to be optimal. Personally I'd say something from 10 to 20 years is probably enough, or maybe different terms for different categories like newspapers being protected for a week and movies for 30 years.