If you are asking to learn Arabic as a language and to write well and be well-conversed in it, it'll take 3-4 months while taking classes from a tutor. But if you want to prepare for CSS exam, and you follow a study plan and pattern, it can consume just 20 days to cover the both papers. It totally depends that how much you study, how you study and what you study.

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"But screw your courage to the sticking place,
And we'll not fail." _Shakespeare, 'Macbeth')