never reinstalled on my current system, but ive just had it for a month,ha, but on our families old one years ago (a shoddy littel eMachines system) i'd do it every 6 months, or when the little thing started to get gunked up
1-2 times a year just to throw out the trash. I keep all my data on another partition and other drives. I have a ghost drive set up for re-in takes about 1/2 hour. Then I just run updates. The I reghost to my ghost drive to have the more current updates. I also made a slip stream dvd so I can have a back up incase I change my comp where the ghost won't work.