I don't really follow (or care all that much tbh) about copyright law, but i'm assuming we're talking about how long till something becomes free to the public to use and distribute without approval of/ payment to, the original creator/s?
Hard to say in that case, but I do know the moment something becomes abandonware it should automatically lose copyright status. same goes for games (especially older games e.g nes, n64 etc) since they're not being sold or even supported by the original creator.