Try not to spend more than two hours on one subject in one sitting. Study your hardest subject first - save your easiest for a break. Study on weekends - a few hours will take the pressure off the weekdays. Borrow time, but don't steal it. Manage your schedule and be flexible.
Time is easy to waste and difficult to control.
The key to successful time management is writing everything down. You can make changes later. On another piece of paper list everything else you have to do and want to do. After everything is listed, prioritize your list and approximate the amount of time it will take you to accomplish each task. Of course, you can add or subtract time if necessary.