There's no magic number of hours a day or hours a week a student can practice for the SAT or ACT and be guaranteed to hit their goal scores, any more than there's an exact number of hours someone needs to go to the gym to hit their goal weight. It really depends on the students, and there are so many variables.
At the end of the day, the right amount of prep is the amount a student is willing to do and that won't burn them out. You want them to put in the work and care enough to succeed, but not get so singularly focused on this one thing that they neglect other important things like their grades in school or their extracurricular activities, both of which matter a great deal to colleges in the admissions process.