Study for 50 minutes and then take a 10 minute break. The average person can concentrate for only 50 minutes. After that, the learning curve and the attention span declines. Schedule your study time when you are most mentally alert. Be honest with yourself.
Some people can accomplish more in one daytime hour than two evening hours.
Like everyone, some students have trouble maintaining their attention span during study times. To stay focused, keep a notepad next to you. As you have distracting thoughts and ideas about things you should or could do, quickly write them on the pad instead of worrying that you will forget them. When you've finished studying, look at the notepad and do what you have to do. Schedule time to review and outline each course.