Your going to get a variety of answers for that question.....
It really depends on how comfortable you are with the material that is going to tested on (i.e. how much you recall from your undergrad classes, etc...).
It took me roughly three months to really prepare for the exam. I know that for some people on this forum, it only took a month or two months.....for me, personally, I did not want to be put into a situation to have to retake the exam...and thus studied for a longer period of time.
Good luck.