I know the importance of being persistent in praying for something, but how do you know it's time to quit praying for what you want God to give you?

For example, my girlfriend of 4 years left me and went immediately to another man.  Well, i forgive her, and I pray to God everyday that she may call me again someday to reconcile.  It's been about 6 weeks, and I wonder if it is time to quit praying for her to return to me.

I know a lot of you are saying to just leave her alone and forget about her, but my love is too strong.  

My mind thinks I have no chance with her, but my heart says that I do.

My question is - "When do I know it is time to stop praying that she will come back to me?