Generally, I'd count anything beyond the first 10-15 minutes of a film as spoilers, so if you're ever talking about a movie, you're safest not going beyond that (unless you really want to piss someone off, of course). So don't go on wikipedia and read the whole plot summary to send to someone (but again, it is a really good way to piss someone off, but don't tell them I suggested that).
My ultimate spoiler pet-peeve is the sheer assumption that you will have seen a film
But my ultimate spoiler pet-peeve is the sheer assumption that you will have seen a film - thanks to one specific teacher who saw me halfway through reading Jane Austen's Emma and told me the ending, guessing I'd watched the film before reading it. Which is the wrong order, by the way. Perhaps what's even worse is that this sort of thing happens in the cinema foyers a fair bit. So as a general rule? Don't talk about a film until you're outside the cinema. Especially something as major as Infinity War. One man was even beaten up outside a cinema after shouting spoilers, according to one NME article. Perhaps an over-enthusiastic response - but it just shows how bloody irritating spoilers can be.
There's also the oh-so-ever-annoying world of Twitter spoilers. Whether from the actors themselves, Tom Holland I am looking at you or those pretentious film fans assuming that if you don't see a film within a week of its release, you won't watch it at all - it's major spoiler territory. In fact, before the premieres of the last two Avengers films, Kevin Feige (President of Marvel Studios) and Anthony and Joe Russo (directors) put out posts asking fans not to reveal plot details with the hashtag #DontSpoilTheEndgame. But even those pleas fell on some deaf ears. My best advice is if you're really looking forward to a film, block any hashtags or words associated with it. And music for that matter...
Oh yes, because when a little-known indie film called The Phantom Menace was released back in 1999, so was the soundtrack by an up-and-coming composer named John Williams. The titles of which nearly all revealed major plot spoilers. I won't be meta enough to tell you them, but search the soundtrack up and you'll be surprised how detailed the titles are.
But there comes a time when spoilers are unavoidable. Big blockbuster films normally have a fairly decent run in the cinema, and after a few weeks, more and more people will be talking about it. So perhaps the best spoiler etiquette is to see a film as soon as you can, and keep quiet(ish) about it for a little while. Because when we can all go back to the cinema, the last thing we want to hear is the ending of Black Widow or No Time to Die in the cinema foyer.