Honestly I've found for the most part that those who work 6-9 hours get more done than those who work 10+. Usually those spending 10+ are playing catchup or learning a new technology at the same time.
I recently had an interesting experience interviewing with a startup (which shall remain nameless). During the interview process, we had a nice discussion about sustainable development practices, and I talked about my willingness to put in the occasional long day as needed. When we got to the point where they expected me to work 60+ hours a week for the next year, they cut me loose.
I could have probably done it, but really only 6-9 hours a day, like you say, would have been actually working on what they were paying me to work on. I'm still not sure how a schedule like that could ever have been considered sustainable for anyone who wants to have any of: a social life, family, or professional development time (going to meetups and such).
