I find I am most consistently productive if I work 8 - 10 hours on the weekdays, and 3 - 5 hours on the weekends. I can sustain this for several months without burnout, despite the fact that I work every day. This is more than an average work-week, but it's also not the crazyness of deadline crunch-times. It leaves enough time for workouts and some socializing. Eventually, though I need a few months where I take at least one day off during the weekends.