It’s difficult to find a hard-and-fast rule for a good statue of limitations. If a movie is old, popular, and has entered the public consciousness, then I don’t see a problem with spoiling. (That is, most people tend to hear that Rosebud was a sled or Soylent Green is people well before they see the movie.)
However, if I’m going to watch an obscure 1930s-era movie with some friends for the first time, and one of them spoils a twist in the movie, I’d whine like said crack-addled rhesus monkey.
It makes it difficult to say that after a movie is X years old, it’s no longer a spoiler to discuss it - especially since in the thread that spawned this, someone mentioned that he’s only 24 years old. Citizen Kane may have been available for 60 years, but if you’ve only been able to watch/appreciate movies for a handful of years, it’s a lot harder to see a wide variety of movies.
It’s a littler easier to define it for more modern movies, as it seems most people have seen more current (1980s-90s) movies than classic movies. The Usual Suspects and The Sixth Sense are two movies with great endings, but they’ve both been out a while (1992 and 1998, I think?) and it’s always annoying to start discussing a movie only to hear someone shriek “SPOILERS!” and cover their ears, or get angry that you dared to talk about a movie.
As KneadToKnow said, watching a movie while knowing the ending is a treat - appreciating the craft that goes into setting up the ending is an enjoyment in itself. I still like watching Sixth Sense and Unbreakable to pick up the small hints that lead toward the ending.
Really, there’s not a good rule-of-thumb. It’s fun to rebut a spoiler whine by citing a statute of limitations, but it’s also a shame to miss out on a great movie twist because the schmuck in the cubicle next to yours can’t keep his mouth shut.
(Was there some movie that, at the end, requested the audience keep quiet about the twist in order to preserve it for future fans?)
Just use discretion, I suppose. If there’s a new movie in release, I’ll try to preface a discussion about it with “have you seen it? Ok, great” or something to that extent.
Eh, I’m babbling. Still bitter about having The Ten Commandments spoiled by my Sunday School Teacher, I suppose.