Many jobs require licensure to advance in both salary and responsibility.
This is a very personal choice. It is dependent on your particular learning style, the type of work you have been doing, your life situation and your tolerance to failure. If, like me, you don't tend to retain information very long-term and feel like anything beyond about two months would end up having to be restudied, you might consider some brief overview until two months prior to the exam. I had been in a consulting job where I did a wide variety of work and had a good background in diverse subjects. This prepared me better than some of my more specialist counterparts.