There is no answer to that question. You can work as many or as few hours as you want, and if your practice isn't busy, you may not need to work many hours. If you have a busy practice it may dictate that you work a lot of hours.
Some doctors simply CHOSE not to work hours. There is a doctor who has contributed many times on this forum (NatCH), who practices in a an area with beautiful outdoor landscape and he enjoys his outdoor activities, etc. He has a very unique practice with some partners who each work a light schedule, but that is their CHOICE. I believe that he said he works about 12-15 hours weekly.
I am the on the complete opposite end of the scale. I'm a partner in a large group practice in a more urban setting. We have several offices, cover several hospitals and wound care centers. We also provide services at a few nursing homes (though we have an additional doctor who we "sub out" some of that work to).
As a result of the number of doctors we have and the number of facilities we cover, we also get a lot of referrals, a lot of hospital consults and are called for a lot of emergencies to be seen in the hospital and office. Our offices see a tremendous amount of patients on a weekly basis.
Therefore, it's not unsual for each one of us to put in 10-12 hour days. Each doctor works one Saturday per month and has to take "call" for emergencies/hospitals one weekend per month. Additionally, each doctor is given a day (24 hours) to cover hospitals for consults. However, we all give each other a hand when needed.
Obviously, when you are this busy, there are financial rewards to go with it, but we all work hard and there are no "slackers" in our group. A slacker wouldn't last a week. It's not for everyone, but we created this "monster" and to keep our practice going we have to be available to treat the patients. We will be hiring another doctor soon.
We all work hard and play hard. I go to seminars and hear guys whining that they aren't busy, etc., but then I also hear them complain that they don't want to come into the office early, they don't want to go to the hospital to do consults, they don't want to work past 5 pm, they don't want to this or that, etc. The bottom line is that they are lazy and we didn't build OUR practice by being lazy, and I don't know of anyone who has a busy, successful practice who is lazy.
At this point, all of the docs in our practice can cut back and take it easy, but that's simply not our personalities. Our patients and referring doctors definitely appreciate our dedication.
Despite my practice, the decision is strictly yours. You may be smarter than me or my partners and figure out how to work half the time and make twice the money. But ironically, even on our "short" days, we still seem to find work, so I guess most of us are "workaholics". I have one day a week when I'm done hours by noon, but somehow I still find something to do until at least 3-4, and all my partners do the same. I just feel guilty getting to the gym that early in the day!