It is very difficult to commit the time required for the success of SEO as it depends on the website and many associated factors. A best-guess estimate is based on analyzing a website's current performance and the proposed strategy.
But with the right strategy in place, 6 to 12 months is an estimated period to see the impact of SEO efforts and to justify ongoing investment into growth.
Many factors are involved in the success of SEO like competition, backlinks, content, and website optimization.