Each student learns (and tests) differently so there is no cookie cutter recommendation for all students, but here are some good suggestions for everyone:
1. Take the PSAT in 10th grade. This will really give you an idea of what the SAT will be like the following year (NO ONE will see your grade so it is purely for practice).
2. If you happen to be in an advanced level class in school, you should look to take the SAT II exam for that particular subject in either May (if you take AP) or June (if you are not in an AP class) while the material is fresh.
3. Take a practice ACT (if available) or register to take the actual ACT in September of your junior year.
4. Take the PSAT again in October of your junior year
5. Your PSAT scores will come out in December. At that point compare your PSAT score with your ACT score and see if one is drastically higher than the other. If one of them is, you should definitely opt to focus on that one. If not, you can take both.
6. You should opt to take the SAT or ACT at least twice, if not three times before applying to college. Target SAT test dates should be March, May, October (of senior year) or January, May, October (of senior year) for international students who don't have a March test. Target ACT dates should be April, June, September (of senior year).
7. Plan to take more SAT IIs in June of your junior year (or May and then take your SAT in June).
The SAT was designed as an aptitude test—it tests your reasoning and verbal abilities, not what you've learned in school. In fact, the SAT was supposed to be a test that one could not study for (though this is hardly the case today). On the other hand, the ACT, is an achievement test, meant to test what you have learned in school. However, this distinction between "aptitude" and "achievement" is dubious. There's concrete evidence showing that you can study for the SAT, and as the tests have evolved, they have come to look more and more like each other.
HOW MANY TIMES?
There is no set number of times that a student should take the SAT; however, most students will take the test at least twice. Some students choose to take the test three times, if they have been working hard to prepare and are confident that their scores will increase. It is not recommended that a student take the test more than three times, and it is recommended to only take the SAT subsequent times after doing some form of preparation, since you definitely do not want colleges to see a lower score on the second test.
Hope this helps!