I think somewhere in the ballpark of 10-20 years is probably enough to reward creators. Anything significantly longer than that, and it just incentivizes them to milk their creations forever, and punishes other creators who might otherwise make novel derivative works.

I am also of the belief that intellectual property in general (copyrights and patents) should be subject to eminent domain in the same way that physical private property is, i.e. it should be possible to force the creator to sell it to the public (with fair compensation) in cases where it would be an invaluable public good.