[It's a little more complicated than that, and Apollo being a skeptic, he's not so sure that the 'Twilight Realm' that Khura'inism believed in existed, per se. That one was like heaven and hell basically existed in the same place together, and once your punishments were over with, you could be reincarnated.]

There's some sort of time limit, but I'm not sure why. Maybe the ghost gets tired, and the medium is able to force them out? I think most ghosts are pretty happy to work with the medium instead of against her. But that's just from what I've seen and heard.