My personal idea would be 10 years, but renewable. However, it can only be renewed if new copies of a work are made available in decent quantities. For a book, this would mean a print run of, say, over 1000 copies. Same for reprinting a CD. This means that, while it's still copyrighted, there are copies able to buy and be acquired. And if it's popular enough to still be relevant 10 years later, it should still be being reproduced. This can extend indefinitely to life of creator, as long as it keeps being reprinted or reproduced like this.

Basically, as soon as it becomes difficult to get, you lose copyright if another run isn't made. This works especially great for academic texts, in my opinion, as well as certain types of music etc. And if someone has a big hit, well, it'll get another print run anyway.

I'd give a longer term for artwork and things like that that don't get republished in the same way, with no renewal. Or maybe something like two free renewals for these things after 10 years, to a maximum of 30 years or life of author, whichever is shorter.