I'm much more clear about what I think is not a spoiler than what is. Describing the premise of a movie, or sketching out the broad strokes of its plot, is not a spoiler. If it is, then the biggest offenders are trailers, not reviewers or casual discussants. To me, a true spoiler must undercut a significant effect that a movie is trying at have with its story. The word "significant" is important because, again, there's all kinds of setups and minor plot points that trailers give away freely. So take a movie like PSYCHO (which I'll now spoil):
I don't regard it as a spoiler to say "The main character, Marion, absconds with her boss' money" -- even though there's a bit of suspense Hitchcock wrings out of the "will she, or won't she" of that decision. Technically, revealing that undercuts the will she or won't she effect. But it's a relatively minor effect, which is really more of a setup for the rest of the story. Revealing that is different from telling someone about the film's big reveal that Mother is Norman. Doing so undercuts the entire effect of surprise that the film is going for. The whole point of the film, in a sense, IS that one effect. (There were lobby standees of Hitch urging people to be on time and to not spoil that plot point for others.) Thus the film is literally less effective if someone's been given the answer to the central mystery ahead of time. But if you know ahead of time that a character named Marion steals some money and takes off, the film is no less effective. IOW, your experience of enjoying the film has not been significantly harmed just because you knew that setup. This is generally the distinction I make.
I find I don't care about spoilers as much as most people do, from what I can detect. The only times I'm truly upset when a movie's been spoiled for me is when there is some kind of central mystery or puzzle to be figured out, as in PSYCHO, THE SIXTH SENSE, or THE PRESTIGE. But if it's just someone going "They get away at the end" or "Darth Maul dies" or "Cameron Diaz gets semen in her hair," I'm rarely bothered by it.
I actually don't think critics should feel any responsibility to avoid spoilers. Which is to say, I don't think the onus is on the writer but on the reader. The reviewer's job is to assess the work. A reasonable, rational human being realizes that assessing the work requires discussing the work. So long as a movie review is clearly labeled as "Movie Review," reasonable people know that what lies ahead is a discussion of what's in that movie. But if someone is super-sensitive to spoilers of any kind, that person should probably try to avoid reviews rather than bitch out a critic for analyzing a particular aspect of a film in a review. That's critics. Casual conversations, online and elsewhere, is another matter. There, journalistic/writerly integrity is a non-issue, but good manners is. It's widely held that it is polite to flag if not altogether avoid major spoilers when discussing a film. Frequently I forget, but I try. But yeah, I make a huge distinction between critics producing works of criticism and people conversing casually -- one is about the nature of criticism, the other about good manners.
There should be no statute of limitations. There's always someone who hasn't seen JAWS. (I think "No, Luke. I am your father" might indeed be the one and only exception. Little kids who've never seen EMPIRE or any of the OT films know that Vader is Luke's dad. I think they tell kids that when they get vaccinated.)