@ryanc That's tricky. It's a function both of time, and of the "pressure" of disclosure - the juicier the surprise, the more likely it is to have leaked out into popular culture.
Miraculously, I managed to avoid spoiling The Crying Game for a decade. Ditto Citizen Kane (to Garret's point). But I think that would have been impossible for The Empire Strikes Back, due to the cultural saturation.
In the fediverse, CWs are cheap enough that it's relatively easy to have no time limit on spoiler coverage.