As you're already aware, SEO is divided into on-page and off-page. On-page optimization involves creating and optimizing visible elements of the site including the text on the pages, titles, etc and invisible components like code. Off-page promotion involves increasing the number of links coming to the site through a link building process.

Initially, the on-page optimization amounts to most of the work. A well optimized site, which is static and doesn't have new pages/sections added to it does not change too much. Hence, if the keywords remain constant and the on-page optimization done is thorough, you will not have to constantly change things on it.

However, off-page plays an extremely important part in search engine optimization and will need to be worked on continuously.