The length of time someone spends studying for the DAT depends largely on each individual's preparedness for the exam starting out. Some people have a basis going into their studies, whereas others must "start from scratch". The time that it takes you can vary tremendously and also depends on how much time you have to spend each day preparing for the exam. I would expect that you have to spend at least one full month to start with reviewing content for the sciences.
I recommend that you start with content review for biology, general chemistry, and organic chemistry, using some sort of books to relearn the material. You could also try videos to accompany your readings. For math, reading comprehension, and perceptual ability, it is more about practicing than studying. You will want to invest time in doing practice questions or full-length tests to practice these subjects. You may benefit from math videos to start off if you are really struggling with the problems.
After you feel you have a grasp on the content for biology, organic chemistry, and general chemistry, you should implement more practice tests into your studying routine. Continue to review content, but I would recommend at least one weekly full length practice exam to gauge where you are at, what your weaknesses are, and to build stamina for your real exam. Access to practice questions that are similar to the real DAT is crucial in preparing for the exam. While most study schedules start with an emphasis on content review and end with an emphasis on practice, make sure to do some practice questions throughout your studying. That way you can push your test date back if you realize you won't be ready in time. Good luck!
