@DHL:
I always thought this would be the case.
I don't think you can run out of Haki (lol) but i do think it's power (frequency) can decrease depending on the users state.
Haki has so many questions needing answering.
Like how does Akainu take and recover from a Haki assault and be annoyed that they're Haki users.
Hey, if you can get the Super Saiyan knocked out of you you can be too tired to use Haki. Either you can't concentrate well enough when fatigued or you don't have the energy to maintain it. Decreasing effectiveness seems very likely at the least.
Tons of Haki stuff was handwaved at Marineford, especially all the CoA stuff since its rules still seemed to be forming at the time. Wouldn't be surprised if stuff like Akainu never got answered, but knowing Oda I also wouldn't be too surprised if they did. It's just something I'm not going to stress over for the time being.
@airy-0:
I think that the amount of CoA applied is not directly related to the strength of CoA.
One can produce more potent CoA, but perhaps concentrated on a smaller surface, yet not being able to do let's say a full body haki while someone else can produce full body haki but at a mediocre level.
Maybe someone like Luffy never really cared for increasing his CoA surface too much since he is able to react quickly enough to produce it when blocking, similar to Doffy.
Pica, meanwhile, lacks speed so he just covers his whole body and hope that it's strong enough to block attacks.
And then there's Vergo who maybe can produce stronger CoA than Pica but can also maintain full body CoA.
To me, generating powerful CoA with a quick reaction like Doffy and then instantly dissolve it, is more meaningful then showing off full body haki.
I definitely agree with this last part.
I also agree that the demonstrated spread of CoA doesn't necessarily correlate to its strength, but I also think that larger spreads require more strength. Applying Haki all over yourself or on large objects seems to me like a skill that might not necessarily be core but still requires a certain amount of power. Seems like it could just be a specific technique of CoA that can be learned once a certain proficiency is achieved. In other words, suppose that Doflamingo's Armament Haki is confirmed to be stronger than Pica's but he doesn't know how to use full-body CoA. He could easily learn and it would be stronger than Pica's. Same goes for Luffy.
On a related topic, I'm unsure if increasing the spread of CoA decreases its overall strength. In that case, small and practical applications would clearly be superior.