Do groups only need 2-3 years to cement their legacy?

yep.

I think it was BP who shot up to like top 3 GG after releasing PWF and Stay right? so sometimes when you hit it big early and cement it it can really set you up for the rest of your career

It’s not really how long that decides a group legacy. It’s their impact.

Depends on how great was their peak : Wanna One has one of the most insane peak between 2017 and 2018 and it would be a shame to forget them in the history of kpop. Iz*One right now reached a peak that will be very remember down the line. If you can't reach that high of a peak, you can compensate with a very long and lasting legacy like Shinee.

kpop groups are short lived so yeah

Depends on their impact and achievements, not how long they’ve been active.

Yup

Twice did it in less than 2 years I'd say. Released hits like Cheer Up, TT, LOA as rookies and were (not self-proclaimed but proclaimed by SK media) seen as the new NGG.

All in just about 2 years - while continuing to release huge hits ie Knock Knock, Likey, Signal in the same 2nd year.