Time should be taken for MBA preparation?

Assuming you mean top MBA programs in the US or Europe, then the most important part of your MBA application will be relevant work experience and an upward career trajectory. Hence, quitting your job to study for the GMAT is a bad idea.

For the sake of the argument, assume you have two otherwise identical candidates. One has a 700 GMAT and consistent work experience. The other one has a 750 GMAT and a six month gap in work experience. Who do you think admissions trust more to get a job post-graduation? Who do you think admissions assume to have better time management skills? As a consequence, whose application do you think is stronger?

While the GMAT is definitely important, don't jeopardize your career and application for it. Most people prepare for it next to their demanding jobs and you are expected to do the same. US and European MBA applications aren't like other parts of the world where the highest test scores get admitted. For example, I scored in the top 1% of GMAT scores and still got rejected from several schools. It doesn't mean much and that is especially true if schools don't think they can place you after graduation. With quitting your job for the GMAT, you're signalling exactly that; plus, you're raising doubts whether you can handle the program, because there are a lot of things happening at the same time.

TL;DR: Don't quit your job for the GMAT. Period.

Now that this is clear, here's my personal timeline:

• December: Super motivated, start studying for the GMAT with the goal of taking it in April, thus leaving enough time for a re-take until applications get published around June.
• Jan-February: A little bit of studying, but not being super productive.
• March-May: Get opportunity to work at a signature project and take some responsibility. Don't get much sleep nor time for studying.
• June: Freak out that it's June already. Approach recommenders, brainstorm about essays, try to get in the GMAT mindset again.
• July: Take a week off of work and finally kick it off for real. 6 weeks until GMAT test day and 3 weeks until TOEFL test day.
• August: Cram hard for the GMAT, stop for 3 days to prepare for the TOEFL. Take TOEFL.
• Early September: Take GMAT and receive TOEFL scores. Scored high in both. Be super happy for a moment, but then realize that HBS application is due in a few days, with other schools following soon after.
• September: Finish the rest of my applications, start preparing for interviews. Get rejected from HBS without interview. (F*ck.)
• October/early November: Get some good news, some bad news, prepare and fly around for interviews.
• November: Wait. (Notice how I didn't say relax. Tried that, but failed.)
• December: Wait some more, get rejections. Then finally get admitted.

Was that ideal? Absolutely not. It could have gone terribly wrong, but didn't and that's why I'm a lucky bastard. A lucky bastard who didn't have a life over summer and instead was constantly stressed out. So don't be me. Start in December for real and schedule up to 6 months for the GMAT, so you can concentrate on your applications over summer. If you want to leave enough leeway for more than 2 attempts on the GMAT, then start even earlier. Maybe I could have made it to HBS (or at least get an interview) with proper execution, but a couple of days just wasn't enough for that. I'll never know (but that's okay).

I hope that gives you an idea of what your timeline should ideally be (or what it could be in the worst case). But again, your job matters far more, so get one again.