When you accept who they really are.
I remember the morning I was at work and my then AGF texted me it was over, sent a picture of her and her new guy, and confessed to f*cking other men while she was with me. Three years clear of it, I think it's pretty funny. And the reason why I think it's funny is without being aware of it, she was showing me what she really was.
Of course, on the day, it wasn't funny. But the good thing about that day was I readily accepted that this was what she is. For once, she wasn't trying to hide anything. It was a rare moment of honesty on her part, and one that I'm grateful for. From that day on, I've been
done with her.
When people show you what they are, you need to pay attention to it. And when you pay attention to it, you can make a decision based on what's best for you. Provided, of course, you're honest with yourself about what you're seeing...