I agree but I disagree. There's such a variety in how people handle things, I see people struggling AND breezing through given the same workload.
One thing I had to change was study habits. If I had 2-3 hours between classes before, I would just go online and goof off (as an undergrad) or go take a really long lunch (or go shopping or something). Someone said it above, but yeah it is surprising how much 2 hours here and 2 hours there really adds up when it's crunch time and an exam is bearing down on you.
And to answer the question, I get about 7hrs on weeknights and 9 on the weekend, drops to 5-6 if I've been taking above said extended lunches (I haven't broken the habit in its entirety yet
)