How do I take the official AWS practice exam?
You buy a token for it in the same way as you schedule your real exams: By going to https://www.aws.training/certification, clicking "Schedule an Exam" and following link after link to make your way into the Testing supplier system.
In there, you can buy a token which will be active on your account until you decide to start the practice exam. The time limit only starts counting down when you begin the practice exam, not when you buy the token.
Note that you may have access to free practice exam codes on the "Benefits" screen in the AWS certification portal.
Can I retake the official AWS practice exams for free?
No. And the questions will be the same every time you (or anyone else) takes that practice exam.
Are the official AWS practice exams worth the money?
The exams can be useful as a personal assessment and studying tool. Use them in the last week of your study to assess your preparedness. After you reach the point where you think you are ready, but in plenty of time to study weak areas or reschedule the exam without penalty