I find that I have significantly more than 40 hours of usefulness in any given week. I almost never work 9-5 M-F. I usually show up later, and stay later; in my experience, I feel most awake later in the day and do my best work later as well. Occasionally, if I'm working on a problem, I can get a lot more (efficient) work done keeping at it rather than leaving and having to spin up again on it.

At other times, of course, leaving is the only way to clear my head and find a solution. It all really depends on your working habits; mine just don't tend to be very consistent. :)