Personally, I get a lot less done per hour in the 35-50 hour/week range than I do in the 1-35 hour range. But I've also found that this is mostly true when I'm working on the same project for 50 hours/week. Give me 35 hours of primary work, plus 10 hours on a secondary project, plus 5 hours on something totally different - and I do pretty well. Just me, of course.