What follows are my thoughts. Obviously, it is impossible to give a "one size fits all" answer. Perhaps you can adapt this reasoning to your personal situation. In any case, you should always consult with your academic advisor.

First Question: When Should You Begin Preparing?

Your LSAT score is not as important as your grades. Therefore, my advice is to avoid preparing during the school year when your energy should be focused on your courses. (This is also why in a perfect world, I suggest taking the LSAT in June (and certainly no later than in October). Those taking the June LSAT can and should begin preparing when your exams are over in late April.

Furthermore, I suggest that you take the LSAT the first June that you are free. Get the LSAT out of the way when you have the time. There is no reason to wait until the June of the year that you apply.

Second question: How Long Should You Prepare For The LSAT?

You should prepare until you are both:

1. Actually prepared; and
2. Feel that you are prepared.

It is impossible to tell in advance exactly how much preparation is necessary for any particular person.

But, I think that it is a MISTAKE to start with a presumption of months and months of LSAT prep (although you may actually end up doing that).

My reasoning is as follows.

At the beginning you will find that the LSAT is a very interesting test (I know you don't think so as you are reading this). The LSAT test designers are required to develop exactly the same test, with different questions, every time. A large part of your preparation is to exploit this design requirement. At the beginning you will find that the LSAT is:

-interesting and exciting; and
-you are likely to make rapid improvements

Your experience with the LSAT will evolve from "interesting" to "tedious". Once your experience of the LSAT becomes "tedious", you will begin to make the small mistakes that undermine your confidence.

Therefore, I suggest that you start with a presumption of a shorter time period (3 to 8 weeks) and move to a much longer time period if you find that you need it.

You can always add time to your LSAT prep. You can never subtract from it!