I prefer a flat time period not in excess of a single lifetime. So, like, 20 years.

Reason being: if an old person invents something (presumably) to benefit his family and immediately dies, I think it’s fair to give the kids time to benefit from it.

Shouldn’t be too long, though, since it shouldn’t have a multigenerational effect. As in, it shouldn’t benefit him during his lifetime AND his kids during theirs.