I used to have a community forum based on a not so popular, pop-culture's subculture niche, it took me around two months to get on 4th place on Google ranking. But it owes mostly to the fact that the niche is not that popular to begin with. All I did was writing news articles revolving around what's happening within the subculture, mostly factual news stories driven from non-canon products release, speculations, etc.
I did little to no offsite SEO except legitimately putting links to my website on the niche's authority sites. We had a lot of traffic and activities in the forum, but I had to shut the site down due to a quite sudden change of priorities in life. What I'm trying to say is - it all depends on how popular or broad the niche you're targetting is, and how much time you're willing to spend working on your site.
No one can really pinpoint the exact amount of time needed to reach the top-five on Google, especially when you're not providing what niche you're in to begin with, and how long you've been working on it, etc.