As much as this seems great on its face, I find for myself that I can't consistently work 10 hours in a day without my productivity dropping off. Sometimes, when I'm making good progress, I'll go 10 to 12 hours, but that's not the norm.

When I was in the SF startup scene, I was very fortunate to work in a company where everyone trusted everyone to work with whatever schedule allowed them to maximize productivity. It turned out that 8x5 worked pretty well for me (sometimes with an hour or two nap in the middle, though I don't do that anymore). In the beginning, I worked more hours per week, but that just resulted in me needing to fall back on the lack of vacation policy to take frequent 1-2 week vacations (adding up to maybe 6 weeks per year). When I normalized my regular schedule to 8x5, I stopped needing to do that (although I still place great value on vacation time).

Many of my coworkers had rather different schedules, including one who seemed to be able to work very long hours about 6.5 days a week without much apparent drop-off in productivity.