For most students, preparing for the GMAT takes 2-5 months. Many students take even longer than that, but it's rare to see a student get a top score in less than 2 months.
As Deepa said, you want to put careful thought into your applications, and go through several rounds of drafts and revisions on your essays and resume. That takes a minimum of a month. (And don't do it on your own! You don't necessarily have to pay a consultant for advice, but at least get a friend with strong writing and editing skills to look over your essay. Most people are pretty terrible at writing about themselves in a compelling way).
Here is my general advice:
- If you're targeting Sept/Oct deadlines, you'll need to work on both at the same time.
- If you're targeting Dec/Jan, try to work hard on the GMAT and get it done by Oct. If you still don't have the score you want by then, keep studying while you begin to prepare your application.
- If you're applying next year, just focus on the GMAT for now... but also think about other things you can do to make yourself an attractive candidate: leadership at work or in a charity organization, taking an online course in an area of interest, etc.
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
