Maximum Duration is a course duration where you will assign a specific time period to complete the course. For example, If you have enrolled in the short course like HTML, and it has maximum duration of 1 Months, It means that you can complete this course within 1 Month.
However, you will have access of your course study material, but you will not be able to take exam. Still, if you would like to take exam after maximum duration expires. Then you need to pay re-take exam fees. So, You should complete your course within its duration or earlier than its duration. As it always a good thing to do.