Via David Yanofsky an interesting Bureau of Labor Statistics study (PDF) shows that casual surveys cause people to systematically overestimate how many hours per week they spend working relative to counts that are derived from detailed time-use studies.
That's no so surprising. You've got your job and your regular working hours, but then sometimes you have to work extra. Needing to do extra work is annoying and memorable, while it's easy to gloss over those times when you were notionally on the clock but not really doing anything. The more interesting finding is that longer hours a person works the more he or she seems to exaggerate. People who think they're working 40 hours a week typically work closer to 37. But people who think they're working over 60 hours a week are typically adding 10 or more hours to their real amount of labor time.
Researchers think this reflects a "social desirability" effect where it's considered good to be working really long hours.