I think you'll find that opinions will vary here as to how long you should give your girlfriend (to ease off on the rules) before cutting the ties. Some will say don't ever do it (since polyamory is the only fly in the ointment -- is it really such a little thing?); some will say give her a year? (based on that being the longest amount of time you can stand to wait); some will say don't give her any time, cut the ties now. You have to take some responsibility for your own course in life, and not depend too much on others to have a unanimous consensus about what you should do. We can't read your mind. We don't know how much of a toll this poly disagreement is taking on you. Maybe it's only a minor annoyance and venting about it sometimes suffices to let you smile the rest of the while.
I'll tentatively vote for giving it a year, but I doubt you'll find that vote to be satisfactory, especially since others are saying zero is the right amount of time. You have your self-respect to think of. Are you respecting yourself by allowing your girlfriend to control your love life? This is a question that you alone can, and must, answer.
An alternative approach would be to only talk about the positives that you enjoy in your relationship with your girlfriend. Perhaps doing so would make the poly thing seem like less of a big deal. Sort of a fake-it-til-you-make-it type of a thing. What do you need in order to be happy? is the key question here. If you can't be happy, then everyone involved isn't happy by definition since that includes you. Is it just a bad attitude? Would talking more about the positives help you to forget about the negatives? again a question to which only you could know the right answer.
I've spent well over five years participating a lot on various poly forums, and have seen everything from relationship anarchists to people who find out they can't do poly (or any other kind of non-monogamy) at all. A one-size-fits-all answer doesn't exist when it comes to how many rules one can and should tolerate in one's life. I myself live in a poly-fi unit and as such there are many rules. I have found (over time) that the rules suit me well enough. But someone else in my position might find the rules to be quite suffocating.
So -- are you suffocating, or just twitching a little over the rules? If you're just twitching, then I'd stick to venting now and then and give your girlfriend as much time as she needs. If you're suffocating, then I'd get out now.
I'm assuming you're somewhere in between those two extremes, which is why I suggest giving your girlfriend a finite length of time (I say a year because you felt you couldn't take it for more than that), and if the rules are as tight as ever by then, then you'll have your answer. But all the advice in the world isn't going to supply you with the perfect answer that meets your every need. No answer is perfect in a situation like this. At some point you're likely to have to choose between your girlfriend and your freedom. Which will it be?
But, hopefully she'll come around soon and you won't have to worry about that. We can, of course, hold out for that hope indefinitely ...
Maybe with more information I'll have a better idea of what to suggest? For now I'm just going by what is presented.
Love means never having to say, "Put down that meat cleaver!"