Almost certainly never. I'd guess that most books go out of print (and stop making money) within a decade or two of being written. It's only a vanishingly small fraction of works that are so incredibly successful that they stay in print and continue making the author (or their estate) significant royalties beyond the author's lifetime.

Looking at it this way, I'm thinking that paid copyright renewals are the right way to go. Have the initial copyright last maybe 10 years, then require the rights holder to pay a small fee (something like $50, maybe?) to renew the copyright for another decade. If a work isn't making the author at least $5 a year, it probably belongs in the public domain.