There is nothing like correct time. It totally depends on you preference. Some students are comfortable in studying in night and some prefer morning time.

Generally, it is advisable to study in the morning as your mind is fresh and you grasp the thing more easily.

Make sure you give quality time to your study not quantity and you should be determined and motivated to crack your exam. You can make your own time table which is convinent for you to follow. So, that you can complete your course easily and you get plenty time for revision.

Good Luck!!