How much reading/preparation would you expect a grad student to do each week? Where I am, we require them to read a book a week. That usually provides enough material for a 2 hour long discussion.
If you’re in a science-based field, you should consider assigning empirical articles rather than literature reviews ; the former seem to inspire more critical discussion because there are procedural and analtyical specifics to hang arguments, criticisms, and questions on (although the latter may be useful as “background reading,” especially for students with less experience in the field). I usually assign 3-4 empirical articles per 2.5 hr class session.
from the CHE fora