How many hours do doctors work?

I've heard many different things about this. I've heard 1/3 of doctors work more 60 hours a week. Is this by choice? How much work is really tacked on when you're "on call"? My aunt is a doctor and she's taken many weeks off - but she has worked for a while. Another works part-time - is that a rarity? To what extent can you really decide your own hours? Is the flexibility available to you right after you become attending?