I think when our actions in a relationship are no longer deliberate is when its time to let go. We need to kind of asses relationships every once in a while to make sure we're getting everything we need and we are doing what we really want. I think when relationships become "just okay" is when it's time to call it off. If you're not sure that you really want it, then it's not fair for either party, if you ask me.
Hope you discover the answer to your problem if that's why you inquired.