When to ask a girl on a date after she broke up with her boyfriend?

There are so many answers to this question I don't know where to begin.
1: How long was the relationship? If it was long and meaningful then there is definitely a cooling off period AT LEAST a couple weeks.
2: Does she still have feelings for this other guy? If she doesn't then I say go for it, but if she does you need to just be there as her friend because right now is when she will need your friendship the most.
3: Do you have any reason to believe she will return your feelings? If not then do you really want to risk your friendship? But if you believe she does then ask her and don't look back!

Horse got to be calm before you saddle her up. Like ashley said, at least couple weeks. Let her do the talking about being ready to move on. Otherwise she'll throw you off. Tough situation as you don't want to wait too long and have someone else sneak on in.