Source: http://www.forbes.com/sites/jaysondemers/2016/02/24/is-seo-an-all-or-nothing-strategy/#544709ac120c
If you continually increase your investment in your SEO strategy, will you also get continually increasing returns? These three dimensions will determine the kind of payoffs you can expect:
Get better returns
Time. With SEO, results won’t be immediately apparent regardless of the amount of effort you pour into it. It’s best to observe the effects as they build over time.
Quality. Depending on how much time and money you spend toward your SEO strategy, the quality of the returns you can receive can improve or become diminished.
Quantity. The actual amount of work you put in on a regular basis has much to do with the results you stand to get. That can translate to anything, from the number of links that help you gain domain authority to the volume of your quality content that search engines can index.