longest I ever went, was about 3 years, with 2 different PC's on xp pro. It ran "ok" by the end of its life lol.
I tend to only bother if i notice a lot of slow down in stuffs, or i get a new OS. Although tbh, I use images now. Build PC, take an image. After a month or so take another image once youve got all your stuffs on. Take periodic backups after that. Then if you notice its gone all down hill you can roll back to w/e time you feel like.