Varies from a person to person. But based on my own experience, even if you study 6~7 hours per day, it has to be with dedication and complete focus of your mind. For me it is rather hard to concentrate on two things together i.e. working on a job etc and preparing for the exam. So if you could dedicate the last 4 or 5 months altogether to the CSS, you certainly shore up your chances of success. Cheers.

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Muhammad Ali Asghar