Luffy probably won't be strong enough to beat an Admiral/Yonko anytime soon, but hes already really close to people like Doflamingo and Marco with G4 who seem strong enough to give the admirals some fight. The gap between Marco and admirals must be really huge If even Luffy who with his growth rate is going to supposedly only be strong enough in the end to beat those guys.
I find it more likely that he will be strong enough in the near future to beat the admirals but it will take him nearly the whole series to surpass the legends.
Unless Oda just makes every arc a lot longer like Zou, Wano, and the Yonko takedown arc then I don't see it taking Luffy that long to beat the admirals.
This is just how I see it as of now so please no flaming.