I've got a bot that de-tags wip's that haven't been modified in over a certain length of time (configurable). I set it for 1 year and ran it on March 7th. Do you think this is too long?
3 months may be best - although you could set it to make comments on users pages after ~2 months to remind them that they probably could update the page.
The consensus was for 2 months. Finding out who put the wip tag on a page may be a bit complicated, so this can't be done automatically. Checking every diff seems impractical. It should show up if it's on the person's watch list.