Sometimes quite quickly. If your site has been around a while and has good incoming links, Search Engine Optimization can deliver very rapid ranking improvements. However, if it’s a new site with no links, it can take several months. This is especially true of Google rankings, due to the “sandbox” tactic Google employs. New sites seem doomed to suffer a “probationary” period of 6 months or more before Google allows them to rise up in search engine rankings.