Absent a legal marriage there is no right of a girlfriend or boyfriend to ask for spousal support. There are however other theories. For example, you may have led her to believe that you would reimburse her for covering your living expenses (rent, food, utilities) and she may have supported you relying on your oral or written representations. There are certain equitable theories that give a person in your girlfriend's situation a remedy, although it is not going to be all that easy for her. First of all, it is very hard to prove oral agreements or representations. Her case will be stronger if she has something in writing. But oral agreements can be proven - it's not impossible, just very hard.
It is also arguable that her support was somewhat gratuitous, that she didn't expect repayment unless you recovered on your accident. So in part it was a gift. A true debt you would be obligated to pay whether or not you recover on your civil suit.
Probably the best thing for you to do when the time comes is to be open and honest. Shoe here what the recovery is, what you propose to pay her, and try to be fair. It is much less likely that she will sue you if she feels you have been fair with her.
The fact that you are even posting this leads me to believe that your relationship may not last long term and you want to know what she will do to you when you get your money and break up. If that is what is brewing really the decent thing for you to do is move in with a friend or your family and don't accept any more support from her.
Leading her on that you will pay her back and all along intending to bail on her does amount to fraud. At least be honest. Don't use her to support yourself if you are not sincerely committed to the relationship. It's just wrong.
A creative attorney can always come up with some theory to sue you. (Fraud, past rent, oral agreement, whatever.) And even though it wouldn't work in other cases, in a situation where it really looks like you took advantage of her - a judge or jury will sympathize with here and you may very well end up paying a judgement. http://www.portlandlegalservices.com