How long do I need to prepare for the ACT if I already have a decent SAT score?

I'm planning to take the ACT, and I'm going to be a senior after this summer. I'm not familiar with the test at all, and I recently purchased the Red ACT book. Since I have the summer, I'm wondering when should I start prepping for the test. I've already taken the (old) SAT and scored a 1940 on it. I'll also be studying to take the SAT again, but I've been told to try the ACT as well. My goal is to get a score that's similar to or better than my performance on the SAT. When should I start preparing? Do you recommend any other sources or good study plans?

The biggest challenge for most students on the ACT is timing. It's much more fast-paced than the SAT. The first thing I'd advise you to do before diving into any sort of study plan is take a timed practice test and see how you do. If you're satisfied with your score, you can set your ACT studying aside for most of the summer and just take one more practice test when you're a week or two away from the test. If you're unsatisfied with your score, go through your mistakes and see which types of questions gave you the most trouble. Then, study the appropriate content, do some more practice questions, and take another full test. Here's a guide that estimates how long you'll need to study based on how much you want to improve. You may improve more quickly because you're new to the test!

The conversion table in this article indicates that a 1940 on the old SAT is roughly equivalent to a 29 on the ACT. However, I'd advise you to aim for a higher score as your goal. There's not much point in taking the ACT in addition to the SAT if your scores end up on the same level for both tests.

As for prep materials, the Red Book is definitely the best resource for official ACT practice questions, so you're on the right track. I'd also recommend getting a prep book called the Black Book, which doesn't have practice questions, but does include tons of strategies for success on the test. It also has great answer explanations for the questions in the Red Book.