The point at which you can no longer date a girl. She has told you too many secrets, or you took too long to make your move.
You are now in the friends zone, and there is no return.
Guy - Hey girl, I've been thinking, we've been getting along pretty good now and I want to take you out on a date, I really like you.
Girl - Awwww I think you're a really nice, sensitive, understanding guy, but I love you like a friend. So, just friends?
Guy - (thinks) Damn I'm in the friends zone.