The proper answer is “as often as it’s necessary, which should be seldom.”

Back in the before-times (XP and Win9x), it was generally a good idea to reinstall Windows about once per year if you were doing anything other than Office (which occasionally needed its own reinstall) and playing Solitaire. Reinstalling Win9x was usually a few times per year thing because it seemed something was always going sideways and it was easier to reinstall than troubleshoot.

XP was really a turning point for Microsoft in the home market and I think a lot of the annual reinstall stuff was just hold-over from Win9x, but I pretty regularly reinstalled XP until Vista dropped.

I never needed to reinstall Vista, but it wasn’t around for very long before Windows 7 dropped.

I never needed to reinstall Windows 7, and I used Win7 from the time it dropped until Windows 10 dropped. The only installations I did were on the original switch from Vista and one system rebuild a couple of years later. I always do a clean OS install when I rebuild.

For Windows 10, I’ve done three installations on my personal machines since 2015. The first was when Win10 dropped, the second was after replacing the CPU, RAM, and motherboard during an upgrade a couple of years later, and the third was a full system rebuild earlier this year.

So unless you’re running Win9x (or maybe XP, but I doubt it in hindsight) or seriously messing with system files (not a good idea), you shouldn’t need to reinstall any of the NT OSs a general maintenance thing. You’d do it if your system drive fried or something became irrecoverably corrupted, but that’s about it.