Preparing for the MPRE
Most Commercial bar courses provide free MPRE courses. The courses provide outlines and lecture materials, strategy lessons, and provide practice questions. These companies have gathered and packaged the relevant materials to make the most use of your time.
Taking Law 609 - Professional Responsibility is not a prerequisite for the MPRE. You can learn the rules on your own, but taking PR first will give you the foundational knowledge for efficient study.
Engage in Self-Testing
You should complete simulated exam(s) under timed conditions for practice until you score in a comfortable passing range (usually 65%). Be cognizant that MPRE multiple choice questions may be dissimilar to past multiple-choice exams. Avoid the "illusion of competency" but proving that you know the right answer through self-testing instead of thinking that you "know the right answer" or "would have" got the question correctly,
Remember, you learn the most by reviewing the answer choices for each question (particularly the wrong answer choices). So look for supplements that contain both the questions and the answer explanations to get the most out of your studying.
Length of Preparation
As with any study-related question, the answer is: it depends. Like all parts of the UBE, the MPRE is a closed-universe exam (meaning the scope of the exam is limited by the subject matter outline). So, start there--how much of the content are you familiar with? Engage in self-regulated learning and self-test.
The NCBE provides information about preparing here.