This post is for most if not all standardized exams.
The number one issue I see with the majority of students who are preparing for these major and critical exams is that they do not spend enough time prepping.
For example, obtaining a tutor a week or even three weeks before the test date is probably not going to do much to increase your scores, especially if you are meeting up with a tutor for only an hour or two per session, given your current score (pre-test).
Here's my take. If you know that you will take one of these major exams (All High School AP exams included), please start months if not at least three months ahead of time. This is when you should start thinking seriously about what you need to obtain a 5 on most AP exams.
What you should do is get a pre-test of how you're doing, thus you would know your strengths and weaknesses as it pertains to the test you plan to take. From here, I would work with the student to create a schedule to fit their time/financial means/test date. It is a great way to achieve most of goals in your life not just used for standardized testing.
1. Get a pre-test completed (it does not have to be the full-length exam, just a shorter condensed version of the test you're planning to take).
2. Get a thorough analysis of your score - not just what you got right or wrong, but also, why you got problems wrong, are they due to careless, lack of knowledge or misunderstanding some concepts you took for granted, etc. Also, check on timing to see if that was an issue, and lastly, types of problems you get wrong. Standardized exam are just that, meaning they are based on a pre-determined type of questions. So focus on say Geometry if that is your weakness.
3. The honest truth is all of us have budget, so take this into account, talk to your tutor and explain to her or him, this is what you have to work with. It is OK if money is an issue, just make sure that means more work will have to be done by you, meaning, ask the tutor for homework that you can do prior to meeting up, thus, use the tutor time wisely and economically.
4. This is one is on the student's shoulders. Yes, most of us are self-motivator, so we can motivate and stick to a semi-rigid schedule for months but not all of us are built this way. Some of us, and you students know who you are, need the constant guidance, thus, make sure your budget and your motivation level aligned. Yes, would be nice to just have to meet with a tutor for a few sessions with the idea that you will do majority of the work on your own but deep down, you know you won't have time outside of the session or simply won't touch anything, which ends up making us tutor feel bad and then look bad.
5. Schedule are great but they also have to be constantly managed, thus, our initial study prep schedule have to be constantly modified to fit students need. More time? more hours? more questions on certain subjects or simply go back to the basic.
6. The number plus in all this prepping is that time is on your side if you start early enough. Don't put off till the last months. Start thinking about AP exams when you just start the school year.
Overall, the length of time that a student needs to prep for any of these major exams depends on their current score, what they want their dream score to be and how committed they are to achieving that score.