Whenever you feel that you're ready to date someone. There's no specific time limit.
Why should they care? They're your ex for a reason.. if you've ended things with them, then things are already done between you and now it's time to move on. You've just moved on a lot quicker than he as. He will eventually as well.
It's never to soon
You don't have to wait thats a dumb social conditioning. If it feels right, do it! Who cares what anyone else thinks? Especially if you were the breaker upper, obviously you wouldn't have done it if you didn't want something else right? Maybe you didn't know what that something else was, but he wasn't it. Go for it.
Nothing is too soon. You don't need to be dwelling on the past and be feeling sad and depressed because most of the time the guys don't wait and that if they didn't have someone else already.
You are right yourself first ..
I've heard that if you give yourself a month for every year you were together.