I'm doing a one-year MSc at the moment and am working ~10h/day 6-7 days each week (because I need to do my laundry at some point). However, due to deadlines only partly associated with my course I have to finish the main portion of work 2-3 months before it is actually due, am also working on two publications not associated with my course, teach and try to keep my collaborations alive. No person "only" doing their degree with normal deadlines should have to work those hours, certainly not for 3 years.
To your question: It just happens. I have a term plan which shows when I have to do what and how much time have to do it. That means I sometimes finish an assignment a month before it is due and have it lying around in case I want to revise it at one point. The thing is, once it is done, it is done - no looking back, unless something really good comes up. That means I essentially have more (self-imposed) deadlines and more allnighters.
For me at least it is also not about a 9-7/8pm day. I will sometimes work 12h each day of the week or for two days straight without sleep. Basically, whenever I feel I can do more than usual I will do it and that means I can take it slow when I am feeling a bit down.
Also important: That is my everyday life but it doesn't mean I am locked up in the lab/office 24/7. I go for drinks every Friday and on the odd weekend trip to visit friends living further away, or we meet in a nice hotel somewhere. One of the reasons I accept my crazy schedule at the moment is because it gives me 2 months off in the summer. I was going to add "and because it is temporary" but to be honest, I do not know if it is. I want to keep my collaborations working and that means I will have to publish outside of my PhD programme too.
I also know people working similar schedules who are building their own company or try to publish a book while doing a postgrad degree. Again, a PhD programme should not be 70h/week 52weeks/year. It wont be 40h/week either but I think no-one expects that. It is also a question of your programme - will you teach for example? That is usually 6-10h/week in Psychology. How often will you be away for conferences, visiting trips, forums, etc? That all takes away time from your daily schedule at some point in the year and means you'll have to make up for it in everyday work.
I personally find that when you are doing something you love, it makes it all seem more bearable. It is still exhausting but in a good way - like having a really good gym workout. All you want to do is shower and go to bed but darn, does it feel good! Of course, it is also easy to overtrain, so be careful. No degree is worth ruining your mental or physical health.
EDIT: What I was trying to say is he might not expect you to put in 10h/day for your degree but all extra-work considered, that is what you might end up with.