ACT and SAT scores are valid for five years. You can resit them multiple times, and some universities will combine the highest marked sections from each test date, known as superscoring.
However, each standard exam costs around $100 (Subject Tests are an additional ~$60), and universities will view multiple retakes critically.
You should take the exam(s) when you are most confident you will score as highly as possible, and in time for your scores to be considered by the universities to which you are applying.
Registration deadlines are typically five weeks before each ACT or SAT testing date. You must register before these deadlines to sit the exams. Otherwise you will be put on a waitlist and admitted to the test centre on a first-come, first-served basis if seats are vacant.
SAT test dates
There is expected to be one SAT testing date for international students expected in each of these months, although official testing dates should always be confirmed via the College Board website:
SAT Subject Test dates are available in:
SAT Subject Tests for languages with listening are only offered in November. You cannot sit the SAT Subject Tests on the same date as the SAT itself.
ACT test dates
The ACT is expected to have two consecutive testing dates for international students in each of these months, although official testing dates and test centre availability should always be confirmed via the ACT website: