I'd actually say go earlier rather than later. Most people like to get a fixed idea of their relationship to another person - i.e. is that person a friend, a potential lover, a person they hang out with who they might have a once-off with, etc. Some of these roles are mutually exclusive - e.g. a lot of people like random sex, but feel uncomfortable hanging around people who they've had random sex with in the past. Hence there's pressure to work out what role to play, and then stick to it. After all, there's conversations and behaviours that she might have with a friend, that she wouldn't want to have with someone she's going to go out with.
That's why you get the scenario where someone may fit the other person's idea of goodlooking (they may have even become friends initially because she was attracted to him), but after having been friends for a while she won't be interested in him sexually, because she's already placed him in the friendship role. It can seem silly, but think of it this way - if people DIDN'T have fixed roles, where they can put someone permanently into 'the friendship zone', it would become impossible to have friends of the gender that you're attracted to, as you could never establish proper friendship due to the constant possibility of the boundaries changing.
Trouble is, this works against the kind of person who either isn't interested in dating someone they don't already know really well, or is too nervous to do so, because by the time they consider acting, the relationship is firmly established as a friendship one. You say you've known her for a long time, so there's a good chance that this has already happened - in which case sorry, but you've already lost. However, it might also be the case that having gone out with the guy and breaking up with him means that she's at a time where she is re-evaluating who she sees as a romantic option, who she sees as a casual sex option, and who she sees as a friend.
If that's the case, you're best off moving early. The longer you leave it, the more she's likely to need to call upon you as a friend to act like a friend - i.e. provide her with platonic amusement and comfort post-breakup. At that point there's pretty much zero chance of you ending up going out with her.
Thing is, you're entirely correct in being worried about things being too soon as well. She might be hung up on him still, she might be sick of relationships right now, or (worst of all) she might be bitter about the way that relationship went, in a manner that she'll take out upon the next person she dates. That's the problem with 'rebound relationships' - they end up serving as a means for the person to take out their past frustrations upon a new date, rather than having a chance as a proper relationship.
BUT here's the kicker. None of those problems are going to resolve themselves in the timeframe that you've got available. If she's bitter, she might be bitter for the next year. Same if she's hung up on him, or if she's sick of dating. Wait that long, and (a) she'll probably be dating someone else, or (b) you'll be absolutely 100% in the friendship role.
SO my advice is to act ASAP. Ok, don't ask her out in the same week that she's broken up, but don't wait for months either. The situation isn't ideal, but it isn't going to get any better either. And if you've liked her for a long time, I'm guessing that you've passed over many opportunities to ask her out in the past, waiting for a perfect set of circumstances that will never arise.
Better to take the view that the circumstances are never going to be perfect, and might never be 'good'. Also accept that this may go poorly, and that it may well damage your friendship - having said that, it's unlikely to ruin your friendship unless you act like a possessive git who can't get over rejection. 12 months down the track it isn't going to matter that you once asked her out - hell, it won't even matter if she knows you're still interested in her, so long as you stick to the designated boundaries as a friend and don't constantly try to hit on her. So there isn't really a lot to lose by acting, looking at it that way - best to get it done with. I hope this doesn't sound rude, but I've known a lot of people like you, and most of the time they never end up asking the person out. If you wait upon circumstances, rather than simply asking her out now because you're interested in her now, there's a good chance that you'll never ask her out.
