You may start and stop courses as you have time. There is no requirement to take a course all in one sitting. When you stop a course and re-launch, it will ask if you want to resume where you left off, including quizzes and exams. If you lose internet connectivity, or have another technical issue which takes you out of the course, log back in and you'll be able to start right where you left off as well.
NOTE: Please check out Technical Issues and FAQ if you feel there is an issue with restarting or stopping a course.