20 years to make some money, and then we set the work free for the public benefit. If it's good enough for patents, I don't see why it isn't good enough for copyrights.

Either that or some exponentially increasing tax so that Disney can keep their vault. (I'm perfectly fine with them keeping it if they pay some proper taxes.)

It also solves the issue of potentially not knowing when the author died, with a fixed period (the number isn't so important imo so long as it's sane), a work is out of copyright x years after the first known copy was published.