I found a great resource which might be interesting to share here. It presents research on the topic done by others as well as conducted by themselves. The key to me is the table they show, reproduced below:
So, of course, the answer is always it depends, but this at least gives a nice baseline from which to work on.
It really depends on the focus of the class, and what other homework you are giving students.
Certain classes are expected to have more reading. Societal and philosophical theory fields are heavy on reading, as are (obviously) literature classes.
Remember, different students will read at different rates, and the more you cover the less detail you can ...
I think you are probably over estimating what an undergraduate can read.
Things might be different in soc sci, as compared to natural science (where I am), but even for a paper without much in the way of math, I tend to allow, say 3 hours for a student to read an average research article (I takes me an hour, and i've been doing it for 20 years).