Generally the post-show chat starts as soon as the Love Cruise episode finishes airing on the east coast and continues until the wee hours of the morning EST. The idea is that this allows plenty of time for the central, mountain, and pacific time zone folks to join the chat as the show ends in their areas.
Of course, keep in mind these are the loose rules -- the chat rooms are open 24/7, so there's nothing to stop folks from chatting either either before/during the show or until well after it's ended everywhere.
-SB