From Forbes Magazine, May 2014
You simply can’t approach SEO with a short-term investment mentality. If you’re looking for a quick hold-and-flip, that isn’t SEO. If you want to know with some measure of statistical certainty what your ROI will be over the next quarter or year, that isn’t SEO. If on the other hand, you’re willing to invest for the long term, if you have a long-term vision and you’re willing to do whatever it takes to reach your end goal… that’s SEO.
Source: http://www.forbes.com/sites/javsondemers/2014/05/21/what-roi-can-i-expect-from-seo/