Hi there... lurker here, and occasional poster...
Right... more whiny relationship rubbish up ahead, so I'll get it over and done with.
There's a girl... still with me? Right, there's a girl, and I've fully established that we like each other. She's told me she likes me. But the problem is that she's been in a couple abusive relationships in the past and is struggling to trust me enough to let herself be vulnerable, since it's been broken enough times before.
I am a patient man, but a part of me would like to know that what we have will lead to something. I've made it clear that she can take as much time as she needs/wants, but I wonder how long is too long...
Do I wait? Do I just try to stay friends and help work with her on her problems? Do I bail? Do I do something else entirely?
Honestly, I would probably recommend you move on. It sounds like what she needs right now is a friend more than a boyfriend. It might not sound like the best proposition at first, but it might be better to just be her friend. Remember, we all need a friend of the opposite gender to remind us not to give up on them entirely.
At the very least, give her some space. She must be going through a lot at the moment, and the last thing she needs is someone pressuring her to go out with them.
Be there for her, and wait as long as it takes. You like each other, and she seems like she wants to trust you, so make it easier for her by not putting any pressure on her. It may lead to something, and it may not, but the only way you'll find out is by waiting around and helping her through what she's going through.
As said above, be a friend first before a lover. Can't really give any real advice based on what you've said, but (not trying to sound demeaning) going to someone IRL for advice should be a first course of action.
There isn't a set amount of time she needs.
If you like her as much as you say you do, you'll find the patience. If you get bored after a few months then you obviously aren't as interested as you initially thought.
Take it at her pace. Be her friend for now until she's ready to take it further. It might take weeks, months, but she'll build up trust with you. The fact it's happened more than once will mean it'll take her a bit longer to feel comfortable with you.
I can't speak on her behalf because I've never been in a physically abusive relationship (One boyfriend tried to beat me so I pulled his fingers out of his sockets. Don't fuck with me...) but I've been emotionally abused and that took me a few months and a lovely trustworthy guy to move on from it. Of course every person is different.
As long as she knows you're a nice guy and have her best intentions at heart, I don't see her taking too long to let you in.
EDIT: I should stop giving advice when I'm in a shitty mood, it just sucks >.<'
If you like her and she likes you, then you will just have to go with the flow. She is going to need time, and your relationship will be progressing at a baby-step sort of pace, but she is not going to trust you any sooner if you don't want to know unless there is a relationship brewing. You'll just have to play it by ear, and let her know you're ok with going at her pace.