I have a pretty high stress job (desk job).
I asked the exact same question on this board, and then a couple weeks later had to unexpectedly quit work :)
I'm guessing a lot would depend on your overall health, whether or not you're high(er) risk, what kind of job you have, ie: stressful? on your feet a lot? etc.
Originally my doctor said I could expect to work as long as regular pregnancy, but that it could change at any second. At 23 weeks I started spotting and she ordered me to quit work immediately. I was put on bedrest and after a couple days off my feet, the spotting stopped. I asked her what would happen if I didn't follow her orders and she said something to the effect of finding a new doctor (she's really great, and I don't think hyper-vigilance is necessarily a bad thing in twin pregnancies).
Anyway, I've not been permitted to go back to work. And after being on bedrest for a short while my employer actually said they didn't want me back (only because they were scared something would happen). But my job was also very physical and a bit stressful. I was on my feet all day working with a young boy with special needs.
So I went from expecting work to like 30 weeks, to being unable to at 23. It really will probably be a play by play situation for you as well. HTH!!
With my first twin pregnancy, I also had a high-stress full time job (worked in politics). I arranged to leave at 30 weeks, and was sooooo glad I did. That really was the point for me where I just needed to rest more. My OB put me on "modified bedrest" which was enough to allow me to take short-term disability for the remainder of my pregnancy (which lasted 38 weeks).
I would encourage you to try to make similar arrangements... I can't imagine having to focus on a high-stress job any longer than that. At that point, it's just time to focus on the babies and keeping yourself as healthy as possible.