I re-installed Win7/64bit from the scratch on my newest PC week after purchase, just because I wasn't happy with the default two partition setting (I wanted one big partition). Maybe I could have merged the partitions without re-installation, but I thought what the heck, got to verify the recovery process at the same time.
I might also want to reinstall my work Win7 system pretty soon, just to see if I can get rid of those "machine jamming with light-blue screen" problems that happen sometimes. I hadn't seen them for a long time, but now as I re-installed Firefox, they are back (I've had it like four times this week already, before that probably not for months). Somehow I felt also earlier they might be somehow related to Firefox, but then they may occur even if I don't use run Firefox. Go figure.
Magnitus: With Linux, not so far.
With Windows, every couple of years.
With Linux, I think I'd need to re-install sometimes simply because I become unhappy by where my distro of choice is suddenly heading, and decide to jump ship to another distro. :)
And I don't remember with which distro it was (could have been Fedora), but at one point I think they warned that in order to move to the next major release, it was advisable not to upgrade directly, but reinstall the new release from a scratch. Possibly something about there being not enough room in some mini-partition for the upgrade or something... Then again, I am unsure if that case is more like moving from e.g. WinXP to Win7, rather than just maintaining the same release.
It is a bit hard to make direct parallels with Linux and Windows world, as the release cycles seem to be so different. XP has been around for more than a decade, while new major Linux distro releases seem to come out every two years or so (and support for the older releases also seem to end faster).