Which one do you refer to? The Rapid is a multiple-choice test, which is not too hard to take.
The Advanced exam is a little bit more challenging than the Rapid, but you can manage it with I would at least 12 months fulltime work-experience as developer. ;-)
You should able to show, that you can work with the whole set of tools (Studio Pro, Sprintr..) and follow all the best-practices they teach you on the Academy courses.
I’m also preparing for the Mendix certification. My colleagues have already done these certification. It is not so difficult. Just go through the learning path https://gettingstarted.mendixcloud.com/link/path. Some mock tests that are available on every module are completed in the learning path and there’s no additional mock tests are available on the internet for free and if you really want to pay and give your mock tests then you may go with Udemy where you have paid mock exams https://www.udemy.com/course/mendix-rapid-developer-practice-tests/. By this, you can even evaluate yourself on what rank do you stand.
All the Best !!