My breaking point is when I lose touch with my friends. I work in an industry that has two major crunches a year. I know that's part of what we sign up for and I manage to work around it. But leading up to a big release a few years ago I just disappeared. I missed every social gathering, and was slowly losing contact with my group. They were all really understanding, but that's when I drew the line. I'm pretty good at managing money vs time tradeoffs - if I'm working round the clock, I throw money at healthy food so I'm not eating garbage. I jog at work so I don't turn into a lump of jello. But friends I can't throw money at, and no job is worth missing out on the people I love.
I went through a bad stretch a few weeks back where I didn't see any of my roommates (who are my best friends) for 10 days at all. Not even a passing hello. I would get up earlier and come back after they all went to bed. That put things in perspective, but only after my friend that I work with told me that I was being an idiot and that I should just go home already.