The amount of study you need depends on where you're scoring now, and where you need to score to get into the schools you want to go to. If you're scoring 160, and you only need a 150 to get in, your required study hours are zero. Without knowing you or your stats, there is no way to tell you how long to study.
I will, however, encourage you to go for the highest score you can get. Even if you are geographically attached to a school, high scores make for good scholarships. That being said, I highly recommend taking a class or hiring a tutor. It's a worthwhile investment.