Honestly, when I saw the "rear admiral" I was surprised purely because of Akainu
I believe Koby at EoS will be on the level of Admiral in strength, but he will only be promoted to be one after Akainu is down!
I think Oda will even highlight it that Akainu is trying to hold him down, and the farthest Koby will reach in ranks is Vice Admiral, because Akainu will not be able to supress his fame and strength among the navy to stay in captain ranks or rear admiral ranks
But generally, only after Akainu is eliminated from story, which is final war, I can see the new leader promoting Koby to be An admiral, which again makes me say "after final war is finished"
I would take literally any bet that Koby will be far away from admiral level, when the series ends.
At the very best he will be a vice admiral like Maynard.
Many people forget that the end of the series is near. Koby has managed to become a captain in two years. However, the end of the series is basically a few months away (in-manga time) and the step from captain to admiral is 1000 times steeper than the step from chief petty officer to captain, which already took more than two years for Koby.
People have to get this: Koby is no mirror image of Luffy (talent-wise) and he isn't a rival/enemy of Luffy.
He is the definition of a "normal guy". He isn't overly strong or talented, but he achieves everything through work, work and even more work. That means he will probably achieve his dream of becoming an admiral sometime (!) in the distant future, but not when the series ends. That would be way too fast.
This is even emphasized by his famous lines back on Water7:
In the first part of his announcement he claims that he will be a lot stronger when he meets Luffy & Co. the next time (fair prognosis that happened to turn into reality).
Then he stops and stutters a bit, following it up with the second part of his announcement: "A-a-and someday in the future ... I want to become an a-admiral!"
Key part is "someday in the future". It's a pretty easy message: He will continue to get stronger and stronger, but his ultimate goal is still far down the future, which his current rank also emphasizes.