Joseph Pelrine has some interesting thoughts on sustainable development - he uses conditioning as a metaphor:
Training is divided into strength and conditioning. Strength training is at a higher intensity level (around 80% of the 1 rep mex, for fewer repetitions - normally 6-8), while conditioning is at a lower level (around 60% of the one-rep max, for more repetitions - normally 10-15). Training at lower levels is considered to be a waste of time, since the body is working neither in the aerobic nor anaerobic areas. Some of these ideas and principles may be applicable to the intensity level of a development team. In the worse case, they are interesting as System Metaphor. If the team's one-rep max is their maximum velocity, something between 60% and 80% of that would be a sustainable pace which would also be accepted by the customer. Through training, the one-rep max may (and hopefully should) be increased. Don't know how to apply this to development, though.
Well, there's no hard and fast rule - I think this is something like the line about art - "I don't know much about art, but I know what I like". You know sustainable pacing when you're doing it - likewise, you know a death march when you're in one. I've been on "you'll work 12 hours a day and like it" projects, and that's an attempt by management to push you to the 1 rep max on a daily basis. It doesn't work, and the results tend to be really sloppy. It's somewhat puzzling - management seems to understand that not "just anybody" can run a division, or a company. Why do so many of them think that developers are interchangeable parts?