I have just passed the exam recently so a few tips I can share are. Also agree with Tim, you will not have time to do a lot of experiments so must have some experience under the belt.

1. Have the completed projects ready on your laptop from the learning path – it will help to verify certain questions on the domain, security, microflow. For instance, what is allowed in the loop in microflow? 
2. Don’t ignore agile training in the learning path – it has some important areas related to development & relevant to exams. 
3. Security and XPath is trickiest so better do some experiments on your own after training path.  

The intermediate is a proctored exam with 50 multiple choice questions.

As preparation take the intermediate learning paths at https://academy.mendix.com/ 
Each learning path module ends with a knowledge check. These knowledge checks reflect the type, level and phrasing of questions you can expect during the exam.

Having 4-6 weeks of project experience a longside these learning paths will be sufficient preparation.

As the exam is time limited and proctored; don’t rely on learning paths and documentation during the exam. the exam is about testing your intermediate level of knowledge you have. Not your ability to use resources.

You will like find that if, during the exam, you still need to browser through the academy learning paths, you will run out of time. That’s why you need some experience.