Most clients of HD Marketing will see some sort of movement within 3-6 months. The folks over at Search Engine Journal say to expect it to take 4 months to a year. It is completely dependent upon the geographic target area, level of competition and the amount of search volume for your particular brand, product or service. In some rare cases it can take 18 months or longer.
While results from seo campaigns is of paramount importance, you must also consider that seo is a marathon and not a sprint. Don't look at seo as something that you can do for a little while and then stop once you see some level of results. SEO continues to compound upon itself as time goes on. The best method or practice is to look at seo as a commodity like you would office supplies. It constantly needs to be replenished.