I'm not sure I'd place it by time. I think it's more a consideration of how much pop-cultural osmosis there has been. I wouldn't even think of spoiling someone for the ending of The Good, The Bad & The Ugly even though it's been out for 47 years. On the other hand, everyone knows (SPOILER)the flower girl dies, Snape kills Dumbledore, it was his sled, Vader is Luke's father, Janet Leigh gets stabbed in the shower halfway through the film, and so on. It really depends how much exposure the plot twist gets in the media more than anything, I think.
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