You should see SEO as a long-term strategy. It can take several months to get to any prime ranking positions from an SEO campaign, and rankings can fluctuate on an hourly basis. However, ranking improvements can start within a few weeks. How quickly you get into better ranking positions for relevant search terms will also depend on how competitive your industry is, and more specifically how many of your competitors are investing in SEO, and also your website legacy – what actions have been taken in the past, how old your domain is, and how knowledgeable and trustworthy you are in your field.