Its only 11.43 hours a day if you work on weekends.
I am not advocating this!
I think if you are passionate about your work and work 8 hours a day (5 days a week) at full capacity you are productive enough. I can work more for one or two weeks if necessary (with the adrenalin boost stemming from a near by deadline) but not for much longer.
This is a great point that is often overlooked in the startup work environment. In 8 years of working at Startups I have only ever been in one place where anyone actually worked a solid 8 hours (everyone happened to pair-program there, which was one of the biggest contributors to this IMO). In most places people spend somewhere between 25-50% of their time surfing the web, reading personal email, hanging out in IRC rooms, socializing, etc. At this particular startup we really did none of that during the day. We all just felt really strongly about what we were working on, as a team we worked with the business guys to set a strong direction for the near-short term, and for four hours in the morning and four in the afternoon we worked. That was it. At 6 pm we left basically feeling punch-drunk for the mental exhaustion, and we went and followed our respective personal pursuits. The result?
The first place I'd worked where we launched on schedule with no overtime worked, with all of the features that business wanted for the big 1.0 (well, web-app 1.0... Really just a euphemism here).
My point is that while Arrington goes on about working insane hours, everyone always seems to overlook how much time in the office isn't actually productive for the product. I've been at places where people "worked" 16 hour days, at least 6 of which were spent playing Starcraft LAN games.