I'll answer in such a way as to incorporate the difference between my undergrad and grad experiences; hopefully that will help justify keeping your question open and relevant. However, I'm trying to keep this relatively objective, because you don't want to get flagged for being too opinion-based.
I probably spent very little time reading as an undergrad, except of course when a test was around the corner (it's a fixed interval "reinforcement" schedule; this is to be expected). I could usually get away with just studying all night the day before a test (that might work out to about 10 hours/month/class), but as a psychologist, I've since learned that this is a pretty poor way to study (at least in terms of how effectively you'll learn), so I wouldn't recommend it, unless it's the only way for you to balance your need to learn with your need to do other things with your life. There's something to be said for finding your natural style early and not fighting it too much just because it's not the best by some specific criterion like information retention.
As a graduate student, my very first class seemed to make a point of swamping me with reading. It was ridiculous, and somewhat infuriating. Lest this seem like I'm diving further into personal anecdotes and opinion, note that I'm not the only one who felt this way, and I know my program wasn't the only one to adopt this sort of pseudo-hazing strategy, which I once heard referred to as the "Harvard Method." (Of weeding out the not-so-die-hard students I assume?) Can't find any internet info to corroborate this, but I'd love to hear if anyone who actually went to Harvard—like the professor of this class of mine did—noticed whether professors like to assign entire books to read on a weekly basis there.
My later classes weren't quite so bad, but I partly perceived them to be better because that first professor was really over-the-top with tremendously long reading assignments due every week, and I had to learn how to skim effectively just to "survive" it. In retrospect, that was a good lesson (albeit learned the hard way), because my graduate studies involved much more reading in general, and more in-depth, weekly accountability. I couldn't just cram from a textbook for a test every month or two; I had to read (skim) enough of multiple separate articles (often denser, with disparate writing styles, and not uncommonly longer than your average textbook chapter) to get a sense of what I could talk about in a seminar-style discussion with much fewer other students to hide behind, all of whom knew my name, as did the professor. I definitely did more reading as a grad student; anywhere from 3–15 hours/week/class when I had classes (not counting reading to prepare as a TA, or studying for my own research purposes). I suspect these experiences are fairly common, but I don't really know.
Other factors affecting my reading time included how interested I actually was in an article, and how messy my personal life was on any given week, of course. I should also add that I sometimes felt I needed to study less than the average student, yet I also felt that I read a given amount of text more slowly than the average student, so I may not be the best representative (probably a multivariate outlier in any case). Empirical summary statistics from educational psych literature might be of more use to you, though I don't have any sources on hand.