for the first attempt 6-8 months give you ample time for preparation. however, preparation should not be haphazard and to the point. also make a schedule and stick to it. for example, make it a point that you will prepare a paper in twenty days. of those twenty days, you will have to devote 15 days to reading the prescribed course and five days for revision, including solving the 5 years papers from memory. you may study for 8-9 hours a day and relax, eat, sleep, watch tv in those other hours.
the point is that you need to stick to a schedule and not waste time that you allocate to your studies. for the remaining time you can do whatever you want.

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Mahmood Khattak
Pakistan Customs Service
36th CTP