Of the last three serious relationships I've been in, only one ended with us remaining friends. We still talk on the phone every couple weeks just to catch up on things. We were together for ten years.
The one prior to that lasted thirteen years and I have nothing at all to do with him. No friendship there.
The most recent relationship lasted three years. It wasn't as serious as the others and we never lived together. I broke up with him a year ago and we tried to remain friends. About two weeks ago I had to cut off contact because he was still being a verbally abusive jerk even after all that time.
So... 33.3% of the time I can be friends with an ex.