I am now a 1st-year PhD student. I spend at least 12-14 hours per day on my course work, and this doesn't even take research/ RA work into account. Acutally what drives me crazy is not time pressure; it's the psychological stress.... when you get stuck with data issues or cannot generate great research ideas, it's really stressful. And I know that most of my classmates work harder than I do.
BTW, my undergraduate major was also business, and I already have two master's. Still, the work load is so pressing for me, and I still have to invest at least five years to get my doctoral degree.
I complete agree with what jadelo said. I spend about 4 to 5 hours attending lectures every day. Then the rest of time is allocated to solving home assignments, reading book chapters or discussion with peer students. Ok, I can sleep about 7 to 8 hours per day. That is it. Besides, in first year, most of coursework is math orentied or, even, pure math courses such as Real analysis, Measure theoretic Probability theory, a bit functional analysis...