I agree it seems common sense that the maximum number of productive hours would vary per person, just as so many other things in life. I would also expect that the number of productive hours is highly dependent on the type of work, the variation in the work, and the incentives people get. Startup work would be very different from other work in that respect.
Lots of people with a 40 hour work week go home to play a computer game for several hours a night. If your work was as much fun as playing a computer game you would have no problem working 10 hours a day. Startup work might be just as much fun as computer games?