Hi srivicool and welcome,
Your question is difficult to answer. It really depends on your target score, English level, and how much time you will spend on your preparation. TOEFL is measuring your English level, and it is not something you can improve a lot in a matter of days/weeks.
If you mean how much time it is needed to be confortable with the TOEFL exam (structure of the exam, what they ask, scoring, etc.) and to do some practice test, I would say 2 or 3 weeks is enough, or even less.
Good luck and I suggest you to take a look in the forum stickies where you will find valuable info.
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