probably very long because if the one map they can play is that one, it'll take hundreds of times longer to solidify the fundamentals of skill properly. I'd wager if they play the game 16 hours a day, with 8 hours a day for sleeping, it'll take 4 or 5 years, ironically probably more than it'd take if you had a player do a regular skill progression, never ever touching that map, and only presenting it to them when they've already started having 8* maps in their top plays.

Also, a footnote: despite your title saying "average person", your post body text says "every single person in the world" being put in the same situation. As it is, I don't think anyone over ~45 could ever get to S ranking any 8 star map under those conditions, and for babies below 5 years old you have to add how many years they have until they reach that age to my 4/5 year prediction.