According to CBP Customs and Border Protection passing rates for the customs brokers exam average only 3-11% nationwide.
Most people who plan to take the exam opt for some training either in a face to face class or online. I took a great class and supplemented it with my own prep process. I estimate that I spent about 40-50 hours on weekends leading up to the exam. I used 6 previous exams and a 3 step system. In step 1 I took each test for accuracy, ignoring the clock. In step 2 I took the tests again in the same order, while timing myself to make sure I could finish within 4 hours. I believe that step 3 was the key to my success. For this phase I circled all the questions I had missed in steps 1 and 2 and created a separate mini exam which I took several times until I answered all the questions correctly. This method worked well for me. I would be happy to discuss with anyone. Good luck!