InfoQ posted the news about Sustainable Pace in XP on May 2, 2008 which title is Does Sustainable pace mean a 40 hour week? That's an interesting topic about agile development. The article focuse on the correlation between the number of work hours weekly and sustainable pace.
As Henrik said, every programmer has a "Maximum Productivity" which depends on his talents. It called "Current Capacity" which is the highest amout of work which can be done whithout lowerring the quality. So he suggest we shoud make sure the every developers' current capacity at the beginning of the project development, otherwise keeping the work week at 40 hours sounds like waste. He said:
If we can hold sustainable pace of say 45 hours per week, as opposed to 40, we gain more than one month of extra development time during a year! Especially in start-ups, such as where I work, 40 hours a week is a luxury one can't afford.
But other guys disagree with Henrik's opinion. They suggested that increasing the number of work hours does not linearly increase the productivity. If we work overtime, teams tend to more destructive than productive. When working long time, the productivity begins to decline.
So the key point is how to value the degree between the number of work hours and productivity. We cannot increase the number of work hours to improve our business value. We must consider about the ability of undertaking the pressure.
The summary of this article is that the important factor to sustain pace is not number of hours worked, but to think creatively and accomplish more in less time. The sustainable pace revolves around a lot of factors like work culture, technical debt, motivation etc. It might have a small influence from number of hours worked, but that is not the best metric to start with.