Spoiler markup should never be mandatory and is more a courtesy for other users who have never bothered to watch the movie, television show or read the book discussed that's been out for some time.
Inside a year is probably about how much leeway you should have. But then it's entirely possible to write a spoiler free title and first sentence or two without revealing anything too pivotal.
If a user clicks through when they haven't seen it, they've only themselves to scold. Why bother trying to answer about something you haven't got any experience in?
If you're going to ask a question about it, and the title has been out for at least five years, then all bets are off about needing to shield eyes. If you haven't seen the film by that statute of reasonable expectation, it's not coming out of your Netflix queue any time soon.