The rule of thumb, which many instructors still tell students is the norm for their classes, requires two hours (some even say three) outside of class for every hour in class. Unfortunately, this rule and reality forcefully collide. The article referenced below contains an excellent summary of various research analyses of student study time. For example in 2003, on the National Survey of Student Engagement (NSSE), only 13 percent of first-year students reported studying more than 25 hours a week for 12 credit hours.