Short enough so you don't get bored and saturated. Take study breaks when it fits into breaks in the material, or when you need them. It is better to take a break than to sit around and feel guilty. Control noise level and the visual environment to acceptable levels. Stop studying when lack of attention or fatigue occurs.
Some students seem particularly unprepared for the academic transition.
Learning how to act and behave like a college student helps students succeed. Don't go to class 2. Go to class late 3. Sleep during class 4. Sit in the back of the classroom 5.