I only wipe when I change hardware significantly, which seems to be between 3 and 9 months. I have no stability problems, but it's easier when making massive changes to reinstall than to reconfigure, and I've got the Arch install down pat now.
Back when I lived with my parents (and thus, had a shared computer), I kept ME running for about 4 years without a wipe, and it had no more problems than ME normally did.