Whether this translates into precisely 40 hours per week at the work site is not terribly important. Different people have different tolerances for work. One person might be able to put in 35 concentrated hours, another 45. But no one can put in 60 hours a week for many weeks and still be fresh and creative and careful and confident. Don t do that.
Overtime is a symptom of a serious problem on the project.
For one week, fine, crank and put in some extra hours. A related issue is vacation. Europeans often take vacations of two, three, or four straight weeks. Americans seldom take more than a few days at a time. If it were my company, I would insist that people take a two-week vacation every year, with at least another week or two available for shorter breaks.