Ideally, you should start preparation in the spring semester of 10th grade. You should at least take a first pass at the test at that point, to understand where your knowledge gaps are and explore how you can improve over the coming year. Many students wait to start prep until halfway through 11th grade, but that can be a bit stressful to balance with school work and leaves them in trouble if unanticipated scenarios arise (we're look at you, 2020). Take it from us: the earlier you look at this, the better.
We sometimes work with students who come to the process very late. For example, students who decide to apply to US colleges at the start of their senior year. While this is very far from ideal, it is not impossible to tackle. We are always open to discussing with you about your circumstances, to advise on what would be best for you.