Right time for MBA
The best time to take the GMAT depends upon two key factors
1) Most candidates rush to take the GMAT between June and November, so it becomes difficult sometimes to get a slot. Choose a time-slot which allows you to get a test center close to your home-base.
2) Choose a time-slot based on your applications calender and also give yourself ample time to take a re-sit if required.
GMAT preparation time depends upon your proficiency levels on the verbal and quant section. If you are comforatbly placed on both, then all you need is practice and one month of dedicated effort should be more than enough for it.
There is no right month to get into US business schools. You need to get an idea about the admission rounds of every business school and decide which rounds you want to target. Although most schools claim that they do not discriminate between application rounds, you are generally better placed by applying early.
Hope this answers your query. Wish you all the very best in your academic endeavours.