It was shortly after they faked out Glenn's death, because I felt the writers were being emotionally sadistic just for shock value and not to enrich the story.
I think I also quit watching The 100 around the same time for that same reason (and because of a couple character deaths that were written very shitty. If you know, you know.)
TWD really was in my opinion the first TV show that had a social media hype train. And the writing quickly felt like a lot of fan service(plot armor and screen time) and shock value (intentional cliffhangers to spur social media conversations)... Other great shows like Breaking Bad and GoT also had huge hype trains but it felt like only connusuiers watched them. TWD was the kind of show that appealed to many generations. It was pretty sad to watch it go downhill. If they had finished strong the IP would be as highly regarded as Breaking Bad.