In our practice tests, it is possible to exit out of the test and resume it at a later date, (when you exit one of our practice test you are given three options, the second of which says "exit the test and resume at a later time,") however we very strongly recommend that you take the test in its entirety. As Mohater wrote, the practice tests are a good way to practice the actual experience of taking the GMAT. They prepare you for having to sit for four hours with two 8-minute breaks and answer questions.
In order to prepare yourself for the exam, however, it is also important to learn the content behind the exam, and to do practice drills and problems. It sounds like you're already doing this on the quant side. To practice just the quant in a more exam-like setting, I would suggest using a stopwatch and timing yourself answering a specific number of Official Guide problems. For example, pick out 20 problems and give yourself 40 minutes to do them all. This is good practice for the problems themselves as well as for timing.
If you have any other questions don't hesitate to ask!
Student Services Associate
Caitlin Clay | Manhattan GMAT Student Services Associate | New York
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