How long should I wait for before i try to get her back?

My ex gf broke up with me about 4 years ago.. We drifted apart from a while. she had a new bf not too long after we broke up. During the past 4 years, we've casually kept in touch, she usually makes the first contact and its usually when things aint going right for her and her boyfriend. I manage to still be there for her and ones its all sorted shes off.. She said am the only one she can trust to talk about how shes feeling. We didn't speak for another year and out of the blue she contacted me again. this time i knew there was something wrong with her relationship hence why she was eager to speak to me. I have alot of feeling and love for this girl so i totally opened up to her one night whilst we spoke. She was so surprise that i still felt that way for her and said to me why didnt i come after her when we broke up and that she misses me. I was careful not to make too much of what she was saying as she might just be saying it because of her emotions. I was delighted i got things off my chest anyways. two days after she told me she'd broken up with her boyfriend and she was hurting so bad. I have just been there for her as a friend because i always have it at the back of my mind that she broke my heart real bad but am still there for her. How long do i have to wait or continue been there for her without spoiling my chance of getting back with her.. I genuinely love this girl. And why would she break up with her boyfriend just few days after i told her i still have alot of feelings for her. any chance of getting back together ?