A new study out of Chicago reinforces what I have suggested in the past and that is that spending too much time preparing to take the ACT is not beneficial. The study found, in part, that studying to much was not beneficial because it took time away from studying the broad content knowledge that students need to know to do well on the ACT.
This is not to say that a student should go in cold to the ACT and just see what happens. At the very least the student should have reviewed the instructions of the exam before taking the test so as to not waste time reading instructions the day of the exam. I also suggest that a student take at least one practice exam just to get some level of comfort with taking this type of test. However, spending a great deal of time in review for the ACT is probably not beneficial.
The SAT is more susceptible to test preparation and I will often recommend that a student do more test preparation for the SAT if they have chosen to take that test. However, this is a point os diminishing return where spending too much time, even on the SAT, is not beneficial.