Probably around 7-10 years, which is the time limit it originally had.
Copyright is meant to be a "public pact" to encourage creation and innovation.
But think about that goal for a minute. It doesn't necessarily mean you should be able to create one awesome thing once and then benefit from it for life as a rent-seeking fat cat.
Instead, it should give you a reasonable amount of time to benefit from the fruits of it, but afterward, you should be encouraged to create again. So you'd be motivated not just by the carrot of making a lot of money over a 10-year span after launch, but also by the stick that you will no longer get royalties after 10 years, so you need to keep innovating. There is even a study out there that shows that the buyers of most books drastically drops after 10 years.
This benefits society at large, since it creates more competition, both from the original author of a work, but also by others who are then allowed to make iterations of the original work.
That's why patents are time-limited, too. The idea isn't to give one company the "right" to make money off an invention for eternity, but to allow the whole society to profit from it eventually by allowing others to drastically improve upon that original idea afterward.
But why was it ever intended as a "public pact" and not like an "actual right" that authors have? Because let's not forget that no idea is 100% original.
In fact, most aren't even 10% original. We all live "on the shoulders of giants" as they say. So most works are just rehashing of old works - so that also means that if enforcement was 100% the inflow of new works would drastically be reduced. So you don't "deserve" to benefit from a "new work" that's actually mostly rehashed old ideas anyway.
I always recommend watching the Everything is a Remix series to get a new perspective on this based on the history of copyrighted works:
https://www.everythingisaremix.info/watch-the-series