It's all about your your BATNA - what's your best alternative? What do you need that they're giving you, and what other options offer it? Do you need the income? How else could you get enough income before you need it? If you don't quit immediately, at least start making plans and preparations now. If you don't need the income - if you need a resume boost or something, there are lots of other ways to do that.

I was in this exact situation last year. I decided I could finish my part of the project in two months if I took on no other work, so I set a deadline and told them I would be leaving on it. Everything got much easier after that - I knew I had a plan, the end was in sight, and they stopped piling on new work, so I was able to finish my part of the project and leave with a clean conscience (of course, your mileage may vary).