As long as you want - infinitely, if possible. Or pick up Nightingale's old "Lead the Field" program, or get an audio copy of Hill's Think and Grow Rich, or Wattle's Science of Getting Rich - or you if you can find one, Haanel's Master Key System.
The reason, as I just heard Bob Proctor say, is that you are constantly growing and so find out more new things about yourself and your constantly improving abilities. He says he's on his 1,000th time through Lead the Field and is still getting more and more out of it. You can take any effective book, like his "You Were Born Rich" and go over it and over it and over it - you may even wind up memorizing it. But the trick is that any book that contains a complete system (such as Hill, Wattle, or Haanel above) you will learn these stable data for your use in life. You will see new parts about yourself and see how what you think and do creates a new Now for you daily. And you'll know the tools to improve your life and have them at ready access.
This is why I created a Student Handbook for the Go Thunk Yourself Series. In that book, I have the student memorize the Summary of the Science of Getting Rich and the Seven Huna Principles. The student also goes through Hill's 6 steps and the entirety of Haanel's Master Key System. By memorizing these couple of pages and 7 principles, the student now has ready tools to evaluate the rest of the universe against. Now the student can compare and contrast what he/she is studying with the rest of his/her experience and earlier training. That student is now able to form their own opinion about the world around them and see if they aren't actually creating that world to a greater or lesser extent.
You can listen or read key and basic books in self-help (again, the ones which contain whole systems - not just a few parts) for the rest of your natural-born life. And you will constantly get new insight from them. Even when you aren't reading or listening, suddenly in your work or play you will see another connection through some datum or law within that book or recording.
Try it. Have fun with this. Forever.