When the thing is popular enough to be a pop culture moment. If the vast majority of people have seen something I feel like they shouldn't have to continually tip toe around open discussions just in case some random straggler just happens to pass by. We shouldn't have to tip toe around the possibility that some random person hasn't seen a 5, 10, 30 year old movie.

I also think context is important. Is it a new movie and your on a discussion board dedicated to talking about the film? I don't think spoiler warnings should apply. If you haven't seen a movie that's actively new and you intentionally go into a discussion about said movie then whatever you read is on you.

Is it a new movie but your on a board that doesn't have anything to do with the movie then take the time to add a warning.