You do not have to take any of our exams in one sitting - you'll receive an email after you complete each section/module with a link to begin the next section/module. So, in theory you can take as long as you'd like between sections/modules (Note: you cannot pause a section/module in the middle and return to it later).
However, we strongly encourage that you take your exam in one sitting, as this is how you will take the real exam. It's important to consider how fatigue will factor into your exam performance, and taking the exam all at once is the best way to simulate this variable.