15 years default, with the option to apply for a 5-year extension twice. In order for an extension to be granted, the copyright holder must continue to make the work readily accessible.

In addition, I would have a clause where if a work is not being made readily accessible even in those first 15 years (delisted from digital storefronts or streaming services, software not compatible with modern platforms, or 5+ years out of print for physical media), copyright is considered to be in a weaker state where piracy for personal/noncommercial use is legal.