Sure they can. Only requirement to stay is that you enter and stay :P Then you get fed all sorts of horror stories about dying or disappearing if you don't stay, or if that's not enough could introduce some sort of enforcement like maybe your bodies starting to fade when you notice that Worldfall is fading from alignment with a visiting world.<br/><br/>They can stay random times, however long you need for that one to have stayed really. Anything from a nanosecond to millenias.