Knowing the format of the ACT test will help. The test has four sections: English, Math, Reading, and Science. There are no negative points for getting an answer wrong. Guessing the same letter or bubble for every questions will get your a score of 11-12 on the test. Now if you can just identify 10 questions in each section and get them correct, you will have a score of 16-18 assuming you picked the same bubble for the rest of the question. So, in short you can finish a 3 hour test in less than 30 minutes and get a score of 16.
Clear everything in your agenda for 3 hours to sit down and take a full length practice test like you would on test day. Once you do this and score the test, you will know your baseline score.
Identify your weakness areas by going through the questions you got incorrect. Calculate the number of questions you got wrong in each section of the test. Now if you can reduce your errors in each section by half, you will be able to raise ACT score by 3 to 5 points minimum. Knowing this set your target score.
If you can spend between 20 and 30 hours over the next month you can improve your score by the 3 to 5 points you identified earlier. You can buy a ACT prep book to review the questions corresponding to the areas your weakness. Start with the English session and spend 1 hour a day for 5 days a week to review and practice questions. Week two move on to the Math section and repeat the same. Week three work on the Reading section and week four work on the Science section.
Pick another practice test and work on it over a 3 hour session. This time, you can employ the same strategy as before but look for the questions that you identified as a weakness and spend a few extra seconds to answer them. If you can get 5 to 10 additional questions correct in each section compared to your previous attempt. This will raise your score by 2 to 4 additional points.