Much less than I should.
When I was an undergrad, I spent a lot of time studying. It was at least forty hours a week. I’d study a lot more for classes I actually cared about getting a good grade in.
As a graduate student, I am basically trying to keep my head above water with all of my assignments, work, and home life. I am basically “learning as I go”. I still need good grades and I should probably study more but it is what it is at this point. Earlier in the semester, I was spending anywhere from 5-10 hours per week on one of my three credit graduate classes. I felt like that was just barely enough.
Sometimes you just need to get crap done and work through the problems without reading the book. You can literally google your way out of problems or just think of how to get around a problem and work it out. Usually I break down a problem into small parts and solve all the smaller pieces until a full solution is apparent.
In undergrad, there was definitely a lot more to learn in a short amount of time as when I went to school I was on the quarter system. I got a mix of A’s and B’s, depending on the class and how it was curved. Most people did really badly in most of my upper-division classes and I was usually a bit better than average.