The answer to this question is not straightforward as it depends on many factors, including your study habits, the time you can devote to your preparation, and the study materials that you select to prepare yourself. Remember, the NPPE Candidate Guide clearly says that you do not necessarily have to use the recommended study materials and that you should get to master the topics of the exam the way that suits you best. Therefore, an absolute time measure cannot be established but a relative one, as a function of the tasks you need to accomplish.
The heaviest part is not just reading the materials you chose but studying them and absorbing information and building organic knowledge that can stay with you and that you can apply during the exam.
If you chose the path of the NPPE list of recommended study materials, your NPPE prep time will amount to the time you need to study at the very minimum two books about 400-pages long each and four other loose articles, plus the time you need to take and review at least two or three mock tests in the format of the official NPPE exam.
However, you can shorten that time if you take the path that suits you best following the advice of the NPPE Candidate Guide. If this is the case, you are probably better off reading a single book that contains all the topics of the exam. (A great candidate surely is “Engineering & Geoscience NPPE: Law and Ethics” by WPE Studio. It is written following exactly the organization of the NPPE blueprint or syllabus. This can truly streamline your NPPE prep and be very helpful to monitor your progress.) Your NPPE prep would then take just the time you need to study a 240-page book and the time you need to take and review at least two or three mock tests in the format of the official NPPE exam.