I do about 50-60 a week. But not billable hours per se -- I am a researcher so that counts thinking time, etc.
I also have a 5 minute commute, which helps.
You should do what works for oyu. I try to spend my work hours wisely -- ie maximizing my time spend doing what is important. But my success is measured by my results not my hours, so that may be easier for me than for others.