I think it really depends on the score you achieve now on your practice exam, and the score you're aiming for.
What I would recommend is that you haven't already, take the ACT practice exam, usually provided in the beginning of study books. Personally, I found Princeton's very easy to understand and straight forward - which is, I didn't feel like shooting myself in the head when I was studying for it.
Then map out how much time you want to allocate to studying based on your score now and the score you'd like to achieve.
I personally found it very helpful just to simply go over ten pages of the book or so every day, completing the example questions given.
When you've finished reading the book, take another practice exam to make sure you've got it down.
If you've made mistakes, look over them. The answer key gives detailed explanations for each answer.
For me, I found that many of the mistakes I made were based off the same reason, so I was careful not to fall into that trap when taking the actual ACT and did pretty well.
Best of luck! :)