SAT II tests in science, math, and foreign languages evaluate skills which take more than one year in a subject to develop. In other words, if your student is required or desires to take a SAT subject test, my advice would be for them to take at least two years (or the equivalent) in that subject for preparation.
We considered registering one of our children for a subject test in chemistry after completing a reputable and advanced (yet introductory) chemistry textbook. When we looked at the practice test on the college board website, we were amazed at the difficulty of the material on the test.
The reason we were considering the test in the first place was because one of the colleges she wanted to apply for required either an SAT II subject test in science or two science courses in an accredited institution. Needless to say, she was not prepared for the subject test. In the end we scrambled and got her into an AP science class online.