These should be used regularly as per requirement i.e. in certain amount of bath time and at night then there will be no more oily feeling on the skin. If you use it for 15/20 days, then the skin may feel oily again. So why not use any face wash, use it regularly and use the cream of your choice. In fact, when you stop using facewash, the skin will be oily again in the sun, sweat and dust, so use these facewash regularly. It is not possible to say for sure exactly how many days it will take to get rid of oily skin.