How long should I keep my fork projects on github?
If I have no interest in being an active contributor or developer, I usually delete mine after the patch has been accepted upstream. I don't see a need to keep the repo around for historical reasons -- the commit will already have my name and email, and the timestamp of the commit, so that seems to be all the historical record it really needs.
It seems from my personal experience that deleting the forked repository also takes it away from the 'Repositories you contribute to'. Just be aware of it when taking a decision.