It's all in the contract . . .
Let's say you join a major health system, agree to work so many hours each week, get your 3-4 weeks of vacation, and take home what ever they have offered. You might take call once a week, give or take. You probably have a good chance of working 40-50 hours each week - a little extra to wrap up paper work, call might bug you once in a while.
Contrast that to being in solo practice, or buy into a partnership. The sky is the limit. If you are free of heafty student loans, or are happy living on a modest budget, I'd imagine you could do just fine on fewer hours. If you have an early retirement plan, and are striving to set the world ablaze, you'll work 'till you drop.
In the university setting, where there is no incentive to work more than required, most attendings put in 40/week. Part of that seems to be spent staring at something on the computer.
In private practice, I've seen attendings put in 40-50. They drive nice cars.
I've also seen guys put in 60, 70, and more. They make serious cash. Too bad they're paying half of it to an ex-wife, and are never home to enjoy the Italian marble in their entrance way.