A) You might apply to even one school that requires them. Virtually all of these are elite schools. A good (but not authoritative) list of schools that require Subject Tests is available at http://www.compassprep.com/subject-test-requirements-and-recommendations/#Req_cat. The only way to be sure about Subject Test requirements is to check each individual school’s website.
B) Your department/major at a college requires, recommends or suggests Subject Tests, even if the college as a whole does not. This is not common but does occur in some math, science, and engineering programs in particular. If something is "suggested" or “recommended,” do it. Here is the list of Subject Test recommendations for UC's.
You should take Subject Tests if:
C) Regardless of any admissions requirements, you should take any Subject Test in which you expect to score very well (~700+). Most schools that do not ask for Subject Tests still look at them if submitted. For example, if you're a native French speaker, prepare for and take the French Subject Test even if you're not required to. It will make you look good.
To summarize from another angle: if none of your schools and none of your programs even suggest subject Subject Tests; AND there aren't any Subject Tests you think you'd excel in anyway; then you can skip them. Otherwise, take them.
2. I'm taking Subject Tests. Which ones should I take?
As with the regular SAT, you can take Subject Tests more than once, so there's little harm in trying a test on which you might do well.
Many juniors who take Subject Tests include US History because they’ve typically just completed the class. Math 2 (which corresponds to pre-calc), science, and foreign language tests offer students who are strong in those areas the chance to achieve very high scores. (Some schools do not accept Math 1; you should check. Also, you can ask your teachers in various subjects if their classes have prepared you for these exams.) Literature is a common exam for students to take and does not have a specific academic lead-in as a math, science, or language test would. The most important message is that you should not guess about which Subject Tests to take. Explore practice questions and test descriptions on https://collegereadiness.collegeboard.org/sat-subject-tests, and try sample tests from books and online.
3. When should I take them?
It depends on which tests you're taking. For tests that correspond closely to academic classes, take them whenever you'll be at your peak of academic knowledge: maybe in June for finals, or maybe in May during AP Exams. Other tests, like Literature and foreign languages, can often be taken successfully in fall of senior year. (Language Tests with Listening are only offered in November. Your counselor can help you determine whether this specific test is necessary.) Most students find that 2 is the right number of tests to take on any given test date. You are allowed to take 3, but expect to be pretty fried at the end.
4. How do I sign up?
Sign up just like you would for the SAT, on sat.collegeboard.com. Students can use an SAT date to take the regular SAT or up to 3 Subject Tests, but not both.
5. How do I prepare? Doing well in the corresponding academic class is the biggest part of Subject Test prep, but students should never take a Subject Test without additional preparation. Subject Tests are comprehensive and typically require a great deal of review to achieve high scores. I am available to work with students to prepare for Literature and Math 2. Please contact me if you have questions about preparing for a specific test!