I used to back in the days of Windows XP, Vista and even 7. But I haven't had the need to since installing Win 10 when I got my upgrades in December 2015.
Yeah same as a previous post, with Windows 7 I used to reinstall Windows about once per year, however with a SSD and Windows 10 I haven't felt the need to do it (yet), so I last installed Windows when I got my new SSD, which is about 1.5 years ago.
Yes, about every two years, usually when I also upgrade hardware.
Even if I don't do a major hardware upgrade, I still reformat roughly every two years because the disk gets cluttered and performance degrades. Everything feels snappier after a re-format.
Thats why I've never done it too. It would be a pain. I've had four work computers that I've had to set up to my liking every time. In the time that I've had one install at home - looks like quality parts do last BTW.