An industry standard for expectation is 4 to 6 months. However, you have to consider that results grow over time. So the longer you are active with SEO practices, the more results you will receive. It is best practice to continue SEO once you begin. For example, you may see your business ranked on the first page, however, your competitors are also SEO optimizing as well. The moment you decide to stop services, there may be a chance that your rankings drop and your competitors jump up in rankings above you.