Depends from person to person, what a person aims for in his exam and also his capability. For example you may be below average in quants, lr and di but you may be average in verbal, so youll end up saving time on verbal.
your question indicates a lack of clarity on your future prospects. id suggest to to make up your mind and get on with what you have to do instead of asking stupid questions.

@Zeenath: As Joshua said, it depends on the candidate but if I talk generally a year time is enough to prepare for the entrance. To begin with, the earlier you start, the better it is.
The amount of time you need to prepare depends how well you have groom

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