It depends on many factors. SEO takes time; at least, it takes much longer than running PPC or display ads. However, factors that determine how fast a website ranks are based on the age of the domain, current traffic, website design, geographical location, competition, etc.
The average time it takes for a website to start seeing results from SEO is around six months. That said, it might take longer for more competitive industries, where everyone is using SEO and jousting for the top position. In industries like these brand-building via social media marketing and PPC management will help level the playing field so to speak till a website starts ranking.