It took me about six months to go back. I did keep in touch with my ex when I started dating the other guy, because he was pretty torn up about me leaving. I still cared about him a lot.
Honestly, the guy I broke up with him for ended up being almost something I'd rather not think about. After we got back together we dated for three more years, and I never talked to the 'other' guy again. Didn't ever want to. He was a reminder that I'd hurt someone I cared about (even though he was nice).