Switching to a gentle cleanser for my acne. How long before I know if it's working?

While I'm waiting for my dermatologist appointment next month, I've decided to eschew all acne products and just wash my face with a gentle cleanser. I've tried several for a few days to a week, but find I still have acne popping up, sometimes in places(like my forehead) that I don't normally get pimples. I was getting frustrated, but then I thought, maybe I'm not giving it enough time? How much time should elapse before I know if a gentle cleanser if helping my acne?