How many months does proactiv needs to be used before the acne is gone forever?

Depends...: On your age, skin condition, etc. There is no fixed time period. If your skin conditions aren't responding, seek a dermatologist for evaluation and recommendations. They may be able to offer better solutions.

Acne: Proactiv is an expensive way to get benzoyl peroxide and is not all that effective. If you wish to use benzoyl peroxide, buy it as a generic at a fraction of the cost. Do not expect a cure.