I would hope that our forces would stay as long as a democratically-elected Korean government wishes them to stay...
I think the case for keeping troops to deter North Korea is overwhelming. If South Korea were still a military dictatorship, then I would think that after the fall of North Korea's government the U.S. should get its troops out: it's not worth annoying the Chinese army.
But South Korea is (and Taiwan is!) now political democracies. And the principle that democracies should be defended against attacks from non-democracies is a very attractive one...
Brad DeLong