I get this one all the time. "I'm taking the test next June... will you be offering a class next May?" Sometimes this question arises from procrastination. But LSAT students are usually a pretty diligent bunch. So more often, this question comes from students who are concerned that if they take a class too soon they'll forget what they've learned by the time their test rolls around. Other times, students are concerned that they need to pre-prepare for their class in order to get the most out of it. My answer is always the same.
The short answer is sooner, rather than later. Take a class as soon as possible so that you'll have plenty of time to practice what you've learned. The LSAT is a lot like riding a bike. It's never too early to learn, and it's very rare that someone forgets this skill once they have learned it.
The longer answer has three parts:
1) No prior preparation is required. At least half of my typical students have never seen an LSAT before the first night of class. This is fine! There's nothing substantive on the test. There's nothing to memorize. All the information you need is right there in front of you. It's a game, and you just need to learn how to play it. You'll suck at first. Everyone does. The best way to learn is just to dive right in.
Students who self-study before the class tend to bang their heads against walls that I could have helped them avoid... they come in thinking the test is harder than it actually is. Sometimes, they get bad materials (like Kaplan or Princeton books) that hurt them more than help them. The process will be much more tolerable with a professional guide.
2) Sometimes it takes a while. It's true that some students take a four-week LSAT class and immediately kill the test--some people take to it more naturally than others. But more frequently, I find that students study for 8 weeks, or 12 weeks, or even longer. The LSAT is only offered four times per year, so waiting until the last minute can be a disaster. I've seen countless students end up waiting an entire additional year to apply to law school because they didn't do their prep properly the first time.
My classes are built to be modular--you can take four weeks, or eight weeks, or twelve weeks, without repeating any tests. You don't have to decide in advance--just sign up for the first four-week class that fits your schedule, and if you like the class you can upgrade from there. Some students decide to stay with the class right up until the day of their test. Other students self-study, after class is over, and continue improving their scores up until test day. (I'm always accessible by phone and email to my former students.)
3) It's not the kind of thing that people tend to forget. Yes, if you stop practicing completely after your LSAT class is over, your skills will deteriorate. But if you're serious about law school then you probably won't stop practicing. And if you know you need the external motivation, then you should sign up for a longer class... one that starts soon, and runs all the way up to your test date.
You're probably not ready if you're currently taking 21 units in your final semester of college, or if you're currently working 80 hours a week. A good class will require a minimum of 15-20 hours per week to study (including class time). Make sure you've got the bandwidth to get the most out of your class. But when you do have time, the sooner the better. A good teacher will save you a lot of headaches, and it's never too soon to start.