Most learning follows a curve; not a bell-shaped curve, but a sudden, steep ascent, followed by a plateau. Think about what it was like to learn how to drive a car. You had to consciously remember where the brake and the accelerator were, until it became second nature. That's the pattern for new learning. What has already been learned is usually habitual, so that it becomes unconscious. When learning new, you have to explicitly pay attention to things that later will become second nature. Then the learning is deeply ingrained; so deeply that you don't even have to think about it.
With regard to exam preparation, this steep curve is a reflection of the fact that you learn the preliminary skill set of exam prep strategies relatively quickly, because there are a finite number of things to learn. The ACT in particular is an open book exam in the sense that all the answers are either in the test or in the questions. It's more a test of your ability to use logic and to read quickly than of your overall knowledge. So it makes sense to start practicing way in advance of your test.
Let's say you have a test date in the fall. If you start studying for it now, in the early summer, you will quickly learn the requisite strategies that can increase your scores by 5-25%, depending on where you are now. But that is only a half measure. If you really want to go all the way, and get into the best college you possibly can, be sure to stay the course. If you learn the strategies and leave yourself plenty of time over the summer to practice them, your ability in logic will become a habit and your reading will go much faster. This means that once you get into college, you will have more free time because you are reading faster than before. If you put in the time to intentionally practice reading for information rather than for pleasure, studying for the ACT/SAT has the added benefit of making your life easier when you have way too much reading to do in the future.
Hence, the best strategy for ACT/SAT test preparation is to start as early as possible, three months in advance of the test, if you can. That way, you can practice the purely mechanical coordination of how you use your eyes as you read until your reading is much faster than it ever was before. Additionally, you have time to learn to pace yourself so that reading the entire passage is a not a matter of forcing yourself to do it, but a matter of habit. It's like training for a marathon. You have to increase your endurance and your mental focus. The dividends are premium, however. Better college, better life, more free time in the future. This makes study for the ACT/SAT a strategy for life, not just for school...Plus, it's way less stressful to push hard well in advance of the test and then taper off as you get close to the exam date. Rest up, drink plenty of fluids, get a lot of sleep, just as if you were in training for an athletic event.