At my school, the counselors recommend taking it during the spring of your second last year of high school. It's also when most kids take it. I took it then and I also took it again and again... . Some really crazy kids take it in December of their year 12 but I think that's a little too much. I'm not sure if your school has it, but ask if you can take the PSAT. The PSAT is similar to the SAT in terms of format and question style, but it is much shorter and a fairly accurate prediction of what you might get on the SAT.
Although you can take the SAT as much as you want, try to only take it three times as a maximum. College admissions officers don't like it if you take the SAT more than three times.