Same exact duration for all authors: as long as they’re around and capable of caring. It’s not the same when duration is expressed numerically, sure. But consider: no life is the same in this way (until immortality is discovered), yet all should be valued equally (or so I hope and believe). Timing life makes them unavoidably unequal - and thus differences in lifetime shouldn’t be a criteria to use for comparing rights. That’d be actual age discrimination.

I suggest to rather look at it under the angle whether estates should be able to inherit author’s identity (or some of it), to continue the will.