The short answer….it depends.
The average time to study for the GAMSAT is 3-4 hours per day for 3 – 6 months. If you have no science knowledge background you might need longer to prepare for the GAMSAT.
The most accurate way to accurately gauge how long you will need is by first spending your initial study time attempting practice exams with no prior preparation. I would recommend using the e-book you receive when you register, there are other resources you can use, find out here. The key thing is that you want to identify where your natural capabilities lie, if you score below par on all sections of the exam then give yourself longer to prepare, it is not uncommon for students to take 6 months to prepare for the GAMSAT.
Related Post: The GAMSAT Study Plan
I spent 4 months preparing for the GAMSAT, the key to achieving a high score was recognising my weakest sections as early as possible. Once I had identified them, I spent most of my preparation time improving my skill and regularly analysing my progress. If you are an avid reader of novels and newspapers and pick up new information in quickly (irrespective of your actual grades) and you have or are completing a science degree, you might be one of the rare students who study for 4- 6 weeks and still end up with a great GAMSAT score!
Related Post: How To Identify Your Weakest GAMSAT Section
Everyone is different, find out your strengths and weakness. Study for the GAMSAT by first identifying your weakest then improving it. For more on my GAMSAT Study plan click here.