A good answer to that...when the conversation between two becomes just that. When someone posts a message, then the other answers back within a minute or so, then that in turn is answered in the same amount of time, usually by that time, unless someone else posts, that's when the discussion should go either to PM, email or our new chatroom...moreso if the posts become more of a personal thing rather than an informative post. A moderator of a sub-forum or administrator may also drop a hint on when it starts to become excessive.
With all that said, we don't want to stop or prevent members from knowing one another or any of that. The more friends you make and the easement of topics and discussions, the more that will make everyone happy.