One thing I will say about this. My father was a college student in 1961. He was taking 18-21 hours each semester. That means he was studying just a bit over or under two hours a week for each hour in class. My son is a college student in 2010. He is taking 14 hours this semester. He took 12 last semester. If he studies 27 hours he is studying just a bit over or under two hours a week for each hour in class.
So, are they studying less? Or are they studying the same amount, but taking fewer classes?
Community College Spotlight has “What’s in a College Credit?” which says that the Department of Education is looking at identifying a credit hour.
One hour of classroom or direct faculty instruction and a minimum of two hours of out-of-class student work for 15 weeks in a semester or trimester program.
So an hour in class and two hours out of class equals a credit hour.
That says two hours out of class studying is/should be sufficient studying. It’s enough to define the term.
Of course, students who are slower readers, need repetition, or just need to take more time ought to study more. I’m not sure that those who are faster shouldn’t study two hours a week. Any course I’m taking I do that for, even if I don’t need to.