when they don't care what their actions might cause you to do
The second time. I had a friend who texted me out of the blue once claiming our friendship would hurt us and that I should move on. I fought for her the fist time and we patched up. The second time she did it. I no longer needed to fight. I knew our friendship didn't matter enough to her to keep her psychotic demons from dictating such stupid texts..
If you are close to the person, don't give up on him or her. Good friends are worth their weight in gold. Discuss how they wounded you and try to sort it out. It may be as simple as a misunderstanding. Without more details I can't give more concrete suggestions.
When you feel the time is right i guess.