Your question is essentially a reframing of the statement that “game balance is mostly an artform rather than an exact science.” Since each game needs to be balanced accordingly to what the general audience playing the game wants.
To that extent… BiS doesn’t even matter. It is just a thing people pursue as a generalized goal. But as it is part of player’s generalized progression goal for their characters, to achieve a ‘real’ sense of BiS:ness you are looking at it taking somewhere in the ballpark of roughly 1-3 months.
If you then want to derive a percentage from that point, you can try to do that. But understand you are still looking at higher ilvl versions of said BiS items as well. This in turn will also increase the amount of time necessary to achieve BiS.
Nothing about this has to do with gatekeeping and you asking this just demonstrates this was asked in bad faith. This has to do with game balance, not “gatekeeping.” The only thing one can safely say is that takes like these…
… is nothing but 100% wrong.
It being obtainable to achieve a generalized sense of “BiS:ness” is already something players can obtain. But a more “true” form of what BiS means? Nope, not even remotely. The number of people who achieve the most “truest” form of BiS:ness, then you are looking at 0.01%. Namely the “1% of the 1%” because BiS across the board at max ilvl isn’t obtained even by the 1% of players.