I wish I had an answer to that question, when is enough enough. To be honest, it's one that DH and I have voiced many times. While we have been able to work through things and get to a better place, we often look back and think that there were times when we shouldn't have. When for our own health we should have separated at the very least. It's a hard question.
The only think I can say for sure is don't let it build to where it's resentment for you and a surprise to her. Let her know that you are trying to be supportive but in the end, other people can not make you healthy or happy. You have to decide to do that for yourself, take the steps you need to. That you want to help her and are willing to, but you simply can not hold on forever, and neither can she.
She needs to decide to get the help. I had a therapist once tell me that "We do what we can to function even if it's not healthy, until we find some way healthier." I've found the problems come when people decide to not find a healthier way. It's hard, I don't think other people can understand it. It's like every day wading through water while everyone else is walking on land. It's feeling and seeing everything through a fog that for some reason surrounds only your brain. It's biking at a resistance at least three levels stronger than everyone else. So yes, some days you just want to stop, you want to give up and you want to just sink and let yourself go because it's too much work. No matter how great a support system, what drug cocktail you are on, some days it's not enough. So yeah she'll have bad days, but it's up to HER to decide to do the work and take advantage of the support system she has. Remind her of that. As calmly and nicely as you can. Because as much as people who love her are willing to take that burden and do that work for her, they can't. She has to.
Let her know, I hate to say it but give it a time frame. Tell her that she has to show she wants the help and to work with you. You can hold on if you see that there's hope but if there isn't, if there's no movement, it's not fair to either of you.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year