Well at a certain point the jaw is going to get sore, and I imagine it'll stop feeling good for the guy eventually too. You could probably muscle through that if your just trying to physically go as long as you can, but everyone's getting diminishing returns for sure
Yeah, this is basically the correct answer. I think the questions really needs to be focused towards whether this scenario is for sexual pleasure or going for some obscure Guinness world record with clear rules.
I guess the scenario would be like edging? Like you have to be actively sucking the entire time. Some quality lungs there. But as others mentioned, the limit comes from the cock at four hours, because after that comes health problems.