When it's not going anywhere. When they don't share the same feelings. When they become involved. And no, it's not as easy as it sounds. It's hard to get over it. I should know, I've been trying for months.
First, does she know that you like her? Because if not then you're gnna have to wait. If she does know, and you are still waiting then, and it has been a long time, then you should stop because she's not intrested.