The real answer is a little vague: “it depends!” It depends on what year you are in, when you are taking the test and what your goal is.
Truthfully, the sooner you start, the easier this whole process becomes. Imagine asking the question, “when should I start working out, and how often will I need to do it if I want to get in shape?” The sooner you begin, the less you have to do on a daily basis to get in amazing shape.
The same thing goes for SAT and ACT prep! Start early and do a bit each day and you’ll be amazing in no time. Start late and you’ll have to do much more each day to get there! It all depends on where you are, where you’re trying to go, and how much time you have – but the one constant is that the sooner you begin, the sooner you’ll be finished, and the less time you’ll have to put in on a daily basis to get there.