I've been reading the Now Habit and it discusses something called the Unschedule. You should figure out all the non-work activities you have on your plate, put them in a schedule, and then fit your work around them. That idea works pretty well for graduate studies who have a lot of unstructured time. I've started something like that before in this post.
It's been a while so I should count hours again. Here's my estimate of non-work activities during a typical week:
Meeting with advisor - 1 hour
Group meeting - 2 hours
Study hall - 4 hours
Class - 3 hours
CMT seminar - 1 hour
Physics colloquium - 1 hour
Medical appointment - 1 hour (I actually go once a month, so this is an average which includes the hassle of commuting to the doctor's office)
Sleep - 56 hours
Meals - 21 hours
Hockey - 4 hours
Gym - 8 hours (I don't actually go 8 hours yet, but I'd like to ... )
Cleaning/computer maintenance - 2 hours
Commute - 6 hours (hard to believe that just walking around campus takes up that much time)
Post office/grocery shopping/random errands - 1 hour
RSS reading - 7 hours
That leaves 50 hours a week to do work (defined as reading for class, problem sets, and research). And it's not possible to work that whole time because you need to take breaks, so maybe at my best, I could do 40 hours of quality work a week?
I'm going to keep a time log of my activities over the next week to get a sense of whether this estimate is correct.