I have seen many times people who drink estus during PvP being called "chuggers", or being mocked or criticized. Why is that? Is it just because drinking estus tends to make the battle last longer?
I've not had a great deal of experience with PvP, but from what I've seen it's so that a fight is as fair as possible.
Even if people are similar levels, one may have found more estus flask shards than their opponent, therefore if estus healing is allowed then one person has an immediate advantage, no matter their build, skill or play-style.
Having an unspoken agreement about using no estus means that duels can occur with minimal discrepancy between the players, and allowing a more balanced fight. It also means that skill is more relied upon, and mistakes are punished harder than just being able to rectify it with a quick chug.
This unspoken agreement also follows on from Dark Souls 1, where invaders (using the Red Sign Soapstone) were unable to use Estus. So, as a "Gentlemen's Agreement", both parties did not use their Estus during the fight.
However, I believe that if someone is invaded (e.g. via a red orb), all bets are off. This is no longer a mutual agreement to fight, as one player now has the advantage of surprising someone who may be attempting to simply play PvE.
There are of course people who will complain either way, and accuse people who have been invaded for not playing fair, despite never having agreed to be invaded. In addition people agreeing to a duel cannot be surprised if they did not establish this rule before the fight began. Most people may simply assume that everyone follows this rule, as many people accept it thus they feel that it doesn't need to be said.