It is kind of hard to say. Have you taken some kind of diagnostic test? You need to know your current level because that will give you an idea how hard you need to study. You also need to know what is the required score to get into the MBA programs you look at.
I am in similar situation like you. I am planning to either apply in fall 2012 or in 2013. However, I do not feel very rushed and that is why I did not take the diagnostic test. I have a lot of work ahead, that is for sure. I plan to take the test before June because the GMAT format will change a little bit so I want to avoid extra studying. However, if I do not do well by June, then I guess I will have more material to cover.
So in a sense you have a lot of time but use it wisely. Think about this: if you start now there is a great chance you will get it all done by June. And when you decide to apply you will not have to lose time on studying and you will be able to make a nice application.
Finally, after you cover all the material download the free GMAT test from mba.com to see your current level. If you are scoring at the level you need then it is time to make a schedule. You should not schedule for GMAT before you have an idea where you stand.
Hope it helps
Set your aims high, carpe diem