Well Rayleigh did imply that using CoC does exhaust a person back on Sabaody. I think he said something along the lines of "With the Marines here, I'd rather not use this ability to much".
So it's not going to be something that people can just use infinitely with zero drawbacks. The amount of time one can spend using it, is most likely dependent on their Willpower, as per Rayleigh stating that the ability grows as the user grows.
Even Luffy who can use it freely, probably can't use it for as long as the likes of Kaido, Shanks, Big Mom and all those others who have been using it for years.
I've said for a long time now, that CoA is not the only Haki that is capped. CoC and CoO should not be any different. People don't have split haki reserves, where only CoA is finite, and everything else is infinite. That's extremely unbalanced, and makes no sense.