Given that Linus has said that he would increase the major number when the patch-level would become stupidly too large, and that happened at .40 for the 2.6 series, we still have that much releases until v6.8:
Let's assume we'll maintain a roughly-3-month schedule between releases, that's 360 months until 6.8 is released. Lets give it a few more months (3) for its .6 stable release, that's a whooping 30 years and 3 months.
So, I don't know how old you'll be, but I should be in my late 60s when linux 6.8.6 eventually gets out... :-/