The short answer:
The summer before junior year. In the meantime, take the hardest classes you can reasonably handle, read lots of challenging material, and work on expanding your vocabulary. Then worry about the test. If you’ve got the skills covered, the actual test won’t be that overwhelming. If you’re weak on the fundamentals, strategy won’t get you very far.
The long answer:
I think that most people recognize that the SAT and the ACT are not tests you can really cram for. Sure, you can memorize a couple of last-minute strategies and rules, but how far they’ll actually get you is debatable. Unless your underlying comprehension of the concepts that these tests actually cover is truly rock-solid and you just need to know about some quirks of the test (e.g. that the College Board considers collective nouns such as “city” or “organization” to be singular), it can be very hard to apply rules you’ve just learned to unfamiliar questions.
On the other hand, it can be just as harmful to start studying for the SAT or the ACT too early. I always hesitate when someone asks me to do serious SAT prep with a student before the second semester of their sophomore year, and I get really concerned whenever I hear about someone who started (strategy-based prep) as soon as they entered high school.
Let me be clear: studying vocabulary and math, and reading SAT-level material are always good, and those are skills that should be built early and consistently. But there’s a big difference between reading a the New York Times op-ed page every night and sitting down with a Princeton Review book; the former will build the sort of vocabulary and cultural knowledge necessary to do well on the test in the long-run; the latter will teach you strategies that will only get you so far if you don’t have the actual knowledge.
I’ve worked with a couple of SAT students who fell into the latter category, and inevitably they were stuck somewhere in the mid-600’s. They had taken dozens of practice exams and knew the test inside and out, but they couldn’t seem to connect the material to anything outside the test itself. They thought that everything was about strategy and memorization, and they lacked (and resisted developing) the flexibility to change their approach based on the particularities of a given question — deadly if you’re trying to crack 700 because this ability is a big part of what the SAT tests.
People who start prepping intensively too early also burn out early; by the time they hit junior year, they’ve had it with test-prep and simply don’t care anymore. Even if they do have the skills to get their score higher, they’re too exhausted to make use of them.
The bottom line is that you shouldn’t make the SAT out to be more than it’s worth. It’s a test — a very important test to be sure — but you shouldn’t let it dictate your life. If you hit 2250, it’s probably not worth it to spend the next six months obsessing over how to get to 2400. Sure you can take it again and try for a higher score, but colleges admit people, not test scores, and all other factors being equal, they will pretty much always take the more interesting applicant with marginally lower (but still perfectly acceptable) test scores over the applicant with super-high test scores and absolutely nothing else.