I am applying for Fall 2012 as well. I don't think it's too early to start.
I would say that you should take a diagnostic test ASAP to see where you are. I scored 690 on my first one without any studying. (The one you get when you register for the test.) My goal was to hit at least 700 on the real thing, and I knew I had to brush up on some of my junior high geometry/inequalities, etc. (Which was almost 25 years ago!!)
I started studying my GMAT prep materials in early February 2011 (I used the 8 strategy guides from
Manhattan GMAT Prep). I work full-time and go to school part-time (still completing my bachelor's), so I devoted some hours on the weekends and some week nights. I decided to take the GMAT in April (last month) so that
if I had to take it again, it would give me enough time to do some more studying and then retake around October. (BTW, I'm not retaking it!)
If you wait until early next year to take the GMAT, that might not give you enough time for a retake (if necessary) in order to apply for Fall 2012. And you probably want to apply in Round 1 or 2, so you'll have to take it earlier, anyway.
Good luck!!