The passing score is not provided by NCEES but after each exam, people share their failing scores on forums. The highest failing score can provide insight into the passing score. The highest failing score that I have seen has been 50, with the majority of failing scores in the range of 40-45. A 51/80 passing score would mean that you need a 64% passing score but a better and much safer bench mark would be 70% or 56/80 correct. If you shoot to exceed 70% then you should put yourself in a good position to pass the exam.
The PE exam is a difficult exam and requires a lot of studying, but it is difficult to tell how much studying is required. The following figure is a survey of 57 people who took the Thermal and Fluids exam in April 2017. The figure shows that the majority of people studied for approximately 100 - 300 hours. This figure does not show the percentage of people that passed the exam for each amount of studying, but you can see the survey in the link below to go through the results.
Every person has their own methods to studying for the PE exam, but there are some similarities between those that have passed the exam. A compilation of these similarities is shown on the links below. Please also read through the material presented on this website for more insight on how to pass the PE exam.