There is no hard and fast answer to this question, because SEO results will vary depending on every client and industry.
It is recommended that any site embarking on an SEO program spends at least six months working at it, as SEO is an industry where the work you (or an SEO agency) does today might not start to bear fruit until six months (often longer) down the line.
SEO success not only depends on the work carried out on a website, but also on what that site’s competitors are doing. For example, carrying out six months’ worth of SEO work on a site will lead to a better website, but it may not improve traffic, revenues, or any other performance metrics if there are 10 other sites in the industry doing the same thing.
It is also important to consider that SEO is an on-going process and that improving your website never stops, so it isn’t a case of “doing SEO” for six months and then never having to carry out any further digital marketing activities. In fact, doing this will cause considerable damage to a website.