The short answer is six months to one year. The long answer is: it depends.
Back in the early 2000s, ranking on Google was pretty easy. You just added your main keywords to the title, URL and content body and boom – you got to the top 10 in the search results within a few weeks or even days. Not for major competitive terms, perhaps, but definitely for the less competitive ones.
The competition was less intense and the search engine algorithms only relied on keywords and links on a 50-50 basis to rank a webpage. This means that if you added lots of relevant keywords to your webpage and built some links, then you had a 100% chance of ranking in Google pretty quickly.