That's a really good question. And naturally, it's not a cut and dried answer.
However, normally there is a lot of upfront work to get a website ranking high in the search engines, and then after a period of time, a lesser amount on a regular and ongoing basis.
This is because the search engines change their search algorithms frequently, and also because your competition typically isn't standing still. They improve their SEO too, so it's always best to stay a few steps ahead.