Think about it, if Luffy uses Gear 4th for 30 minutes, for over use of CoA. If he uses Gear 4th and advanced CoC, the haki usage will be far greater. Only difference here he's learning that while in battle with Kaido, whereas before he was training.
IMO Luffy will have to find a way to maintain Gear 4th without the need to use CoA, Which will become Gear 5th, I for one think he'll be able to do this with awakening. Thereby he'll be able to fight at his strongest without a time limit.
Yamato's going to the roof to stall for 10 minutes in order for Luffy to regain haki. This battle is far from being over.