How long does it normally take for someone to get to the top 20% of a popular speed run?

A popular Speedrun? Well, I'll use Super Mario Odyssey as an example. That game has a lot of runners and optimizations. Each runner learns incredibly precise tricks just to advance into maybe the top 25%. If I had to take a guess, it'd probably take maybe a couple of weeks or a couple of months depending on the game. Super Mario Odyssey would probably take a maybe a month or two? If we're talking Super Mario Bros, though, that would probably take maybe a year or so to get into the top 20%. It really depends on the game and how popular it is.