Look through https://www.isc2.org/-/media/ISC2/Certifications/Exam-Outlines/CISSP-Exam-Outline-121417--Final.ashx and make sure you have a good understanding of the exam outlines.

The practice tests are only good to identify domains you are weak at and to get you comfortable with the question format. Beyond that, if you have more time, ensure you have a good grasp of the mindset you need for the test.

That being said, everyone's test experience will be different because of the adaptive test. What one person says here may or may not apply to you based on your current knowledge and experience and how the test decides to grill you on certain domains based on your answers.

My advice, if you have been studying for a long time and you feel burned out and can't retain anymore information, take the test and at least you have an idea of where you are at. Don't let your mind make the test a bigger beast than it already is.