Is that the only school you want to focus on? No others? Sure?
I recommend:
1. 10-20 days to first research on schools, see if you really know your goals (if you already do, good!), and which schools can help you achieve them. This will also tell you what score you need to aim at in GMAT.
2. Start GMAT Prep. Take diagnostic tests and assess your level. This should govern the time you have to take your GMAT. It could be Decemeber, or it could even be February.
3. If you're taking GMAT in Decemeber, feel free to just continue doing point 1 a little side-by-side, and don't worry about apps. However, if its in January or February, then you should start researching and working on your apps side-by-side with GMAT (NOTE: This is only my opinion -- you need to figure out what fits you best).