I took the ATP written recently and id say 3 weeks prior to the test is just about perfect. You will want to have seen all the questions once so you can do a final review during ATP-CTP. If you start it too soon you'll be sick and tired of going through all the questions. Most of my group scored in the 90% on the test, probably the worst scores were mid 80%. As Sheppard says most of the test is those performance chart questions where you follow the reference lines. It started getting annoying when every other question refers you to a figure and it takes a few minutes to answer each one. All the other questions looked similar to what Sheppard has but there's new questions here and there and the wording will be recognizable but different. The Sheppard thing at least gets you the essence of most of the questions so you know how to answer them on the real test.