Until you have sorted things out with your ex and know where you stand with him - be it friends or enemies. Until you definately know where that relationship has settled, it is impossible to start a new one, because you will still be thinking (at least subconsciously) "Is it possible to get back with him?" - wait until you know for certain if he is (1) An enemy that you never want to see again, (2) Someone that you could be platonic friends with or (3) Someone that you could give it another go with.
I've heard it takes about a quarter to a third of the relationship to get those feelings sorted - so for you, about six to eight months or so? It's a long time but any less and you will still probably questions floating about in your head. But basically, when you have those feelings sorted and an answer as to where your ex now is in your life.
Edit - personally, I think the 1/4-1/3 thing is bs, but the rest I do agree with.