Hi nik1489,
Once you have an idea of which Schools you're going to apply to, you can do a bit of research (visit the School's websites) and find out when the application deadlines are for each application Round. With that information, you can reverse-engineer your schedule so that you can give yourself enough time to work on your applications and enough time to study for (and take) the GMAT. You might even want to give yourself the necessary "buffer" to take the GMAT more than once.
It's important to establish a baseline score early on in the process, so you can get a sense of your strengths and weaknesses and any other potential problems that you'll need to work on (pacing, endurance, etc.). You can download 2 free practice CAT tests (and some additional practice materials) from
www.mba.com. A FULL CAT takes about 4 hours to complete, so make sure that you've set aside enough time to take the entire CAT in one sitting. You'll likely find it challenging, but that's okay. If you want to do a little bit of studying before you take that CAT, then that's fine, but don't overdo it. All of that work will come later. Once you have those scores, you should post back here.
GMAT assassins aren't born, they're made,
Rich