Websites are like cars: they need maintenance. That may come in the form of updating the CMS, security or plugins. The same goes for SEO.
A great SEO strategy will always need reviewing and constant scrutiny of analytics, in order to ensure that you remain on the intended path.
To garner the most success from search engine traffic, a fully-fledged campaign is the best bet, where an individual or team can dedicate the correct amount of time to deliver results.