Never. On my Windows PC, my last fresh install was on Vista, and since then, I have made in-place upgrades and image restores when switching to new system drives.
On macOS, I also never needed to clean install.
Back 20 years ago, I used to reinstall Windows every twelve to eighteen months. There's just no need any more.
Of course, I don't install warez any more, that probably helps. But mostly Windows just turned into a real operating system about 10 years ago.
I admit I've been starting to think about a reinstall, though. My WSL-1 install is so old that the instructions on how to upgrade to WSL-2 don't work, and I can't figure out how to make them work. So maybe a reinstall would do it.