The purpose of copyright should be to allow authors to live off their work, and the purpose of its expiration should be to allow society at large to benefit from it.

Given that, I'd favor a return to the origins of the Statute of Anne, so something like 15 years with the chance of a single extension of 15 more years. Only the original author can apply for the extension, but the rights don't expire with their death and can be sold or inherited.

This would give people 30 years to earn money from their work, and if a business buys them they get at most 15 years to use them, without the chance of hoarding them for too long.

I guess the ideal duration could be more or less than that, but I feel a good rule of thumb is: if some work came out during my lifetime, I have a reasonable chance to see it become part of the public domain while I'm still alive (and potentially use it to make something else from it).