For new websites SEO can take 3 to 4 months of work to get significant visibility and rankings for competitive phrases can take even longer than this. In this regard it is initially a much longer process than adwords for gaining immediate traffic. The reason for this is because Google is assessing the website for it’s usefulness or relevance during it’s early stages and there are many aspects it needs to track over time before ranking them. Google does not make it’s ranking criteria explicitly clear and regularly changes the way it ranks websites every few months so efforts are needed to stay ahead of the game and keep track of when Google will be updating it’s ranking index. This is why most companies will employ an agency to do this for them.
ref: there are over 250 influencing factors that Google use to index a website← What are the disadvantages of SEO?