I guess you mean during the lifespan of the Merit System, requiring gained merit as part of the path to reach the Legendary rank. If that is the case, then strictly speaking nobody has reached Legendary rank formally, since the merit system has been around for less than nine months, and you need just over two years at least in terms of activity to be able to go from Newbie to Legendary (1k needed merits aside).

Nevertheless, some people around have manages to earn the 1k needed merits requirement in this nine month period, although the numbers are few and should be read as an exception and not a norm. You can take a look at this thread to get an idea: [Merit Analysis] "Self Made" high ranked users based on earned merit.

The main driver nevertheless is putting time into your posts and effort. That is how to at least opt to being merited. The speed at which you gain merit, and the quantity, will vary with many factors that are content related (topic, originality, depth, informational, wittiness, grammar command, post placement, etc.). All these factors are important, but come in second place to effort.