A lot do.
I know myself, I easily can average 12 hours a day. Now keep in mind, that is not working a straight 12. I work a solid 8 from 7 or 8 a.m. (in winter. Summer's I am up earlier) and work through until 4 or 5. Taking my 15 minutes for break and 30 min for lunch away from computer. Then I chill for a few hours in late afternoon for dinner. Maybe get out of the office, walk the mall, run errands, mail DVD. Then around 7-8 I come back and work 3-4 all depending on how tired I am.
There is always something that needs to be done, and never enough hours in the day. When I want a half day the next, or the weekend, I will put in more work so I can make that time for myself.
12 hours? No problem. Easily do it, and many other's do as well.
