One topic I didn't really see mentioned was the quality of your work. Being in a science PhD program, I have known both people who have worked their tail off working >60 hours/week, and some who skate by working 40 hours/week. The workaholics have gone onto the more prestigious and higher paying jobs, while the skaters are being reimbursed for their lack of training (very poorly). Why would anyone pursue a PhD if they are not ready to devote themselves to it? Whatever reason you have for getting a PhD, I don't see how someone can be successful without putting in hours, and lots of them.

That being said, there are lots of hidden hours people don't consider. For example, most people read articles in their field during meal times, they also read while at home, instead of watching TV, going to a ball game, etc. When you add this up, I think most people work more than 40 hours/week and don't even know it. But...

One other thing to consider is that you can dream all you want about leaving in 3.5 years... but it will come down to your adviser and your committee. I have a great project and made really good progress, but my committee didn't like my progress and added literally 3 months of work to my time here after our last committee meeting (good news, I just finished my 3 months of data gathering and literature searching and am meeting with them next week to appease them). I would also caution against being care-free in choosing an adviser. Sadly, I know many who do tolerate family obligations. If you get stuck with one of those types, your life could become very stressful very quickly. There are also advisers who have strict work requirements and will micro-manage. Others will never check on you, but will gauge your hours of work by the data you produce. Remember, this is the person who will write your letter of recommendation.