I feel we should all back off at certain times and not waste our times. But every now and then you meet a girl who will throw you enough of a bone to chew on and then ignore you. Let's talk about times when it is better not to waste our time and when it is still worth pursuing a girl i.e. given that your progress with her is not clear to you.

edit:thanks for the answers. It's really appreciated. Also, I would like to add I don't want to talk about a particular girl, even though the question came to mind after a recent experience. The question was, in general when you feel is a good time to back off or stop wasting your time on a girl.