I think this is the longest-lived Windows install I've had in a long, long while. I might be cheating a little by referring to it as one singular installation when it started life on my 120GB Sandisk SSD and not only got moved to my 240GB SSDNow! but also got converted from an MBR to UEFI install, not to mention how some Windows updates are distributed as whole new builds of the OS and install accordingly, but I think it still qualifies. The oldest thing by modify date in my Windows folder is from 8/19/2017 so that's what I'm using as my measuring stick of sorts.
Usually I'll hold onto an install for two, maybe three months at most, then I'll get the itch again and OS hop once more. But now with the sheer scale and amount of the tools and games I install, it's just exhausting to do it all every time I reinstall, so I decided to cut the crap and stop wasting my time. Windows 10 is annoying at times, but more or less I'm happy.
Hurrah for laziness I guess.
I had one installation that was maintained for 7 years through 3 mainboards and multiple versions of TrueImage.
Just changing all the program settings, installing plug-ins, etc. is such a PITA that I avoided clean installs as much as possible.
Of course now MS wants to own all your stuff, and do the opposite.