I reinstall when stuff gets really screwed up and I don't want to bother to figure out what it is. Or every 6 months or so. Depends which computer. Been running the same install on my laptop for about 9 months. My desktop has been reinstalled twice in the past 9 months. Did it just recently because my game kept BSODing. It still crashes and BSODs though even with the new install... I'll install software as needed. It's annoying when I do finally need the software and it's not there, but how often I use it, that will be the only time I use it on that install, lol.
First thing I always do on a new install is get on IE and download Firefox. That concludes my use of IE for the remainder of the time until I reinstall again.