I'm curious about this topic. How long ago should something be published before it's acceptable to spoil something major without warning?
That's rather hard to define because it's not just about time but popularity and availability of the material, and the accessibility of the material for person you're talking to, in addition to good manners.
Is it acceptable to give the main clue to something like Hound of the Baskervilles? How about who Emma marries in that Jane Austen novel?
Well known works of literature that have been around for many decades so no, I wouldn't be worried about spoiling them. However, it doesn't take a genius to see that especially in the case of a mystery like Baskervilles part of the appeal is NOT knowing. And just because a mystery story has been around for 100 years and been made into half a dozen movies doesn't mean that EVERYONE knows the story. I repeatedly pepper conversations about books and movies with the phrase, "Did you ever read/see 'x' ?" That's not just about avoiding spoilers but about being able to understand where the other person stands in their general knowledge of what you're about to start referencing.
For example I've read "Crime and Punishment" but a LOT of people haven't. They know the TITLE but may not know anything about the story. So before I go blathering on about Raskalnikov and Sonya I'd want to know if the other person HAS read it. Even though the crime takes place at the beginning of the book I wouldn't want to start retelling the whole thing and spoiling the potential readers own perceptions of one of the worlds greatest works of literature. If they haven't read it but still know all about it from other sources there's nothing really to spoil.
What about the answer to Who Killed Laura Palmer? How many years have to elapse until you can casually say what happened to the head of Hogwarts?
Some thing applies. Due to their popularity it's not as if I'd feel bad about spoiling key events in such things I'm going to at least try to be a bit cautious if I suspect that A) the other person hasn't seen/read the material, and B) is likely to WANT to see/read it for themselves. Just asking, "Have you read the Harry Potter books?" establishes whether you need to proceed with greater caution regarding spoilers and gives the other person the opportunity to say, "No, but don't spoil it for me!"
I was once engaged in a conversation with my brother about a particular Tom Clancy novel and several friends were standing there with us. Nearby a TV was displaying a football game at a stadium - the same stadium featured in the novel. I made some joke or reference to what happens there which is a HUGE event in the book. Well the book had at that point been out for a few years and in fact one or more books featuring the same characters had already been published. Well one of those friends had not yet read the book and was annoyed that I had spoiled it for him. I felt bad but not TOO bad as you can't be expected to avoid spoilers forever.
If you wait long enough you WILL find out the secret behind the Hound of the Baskervilles whether you want to or not. If you don't go to a lot of movies then like it or not you're eventually going to hear the secret behind "The Crying Game" and "The Sixth Sense" whether you want to or not. Spoilers are just something we all have to live with. There aren't any rules about not giving them other than your own discretion - though that assumes that you HAVE discretion and WILL exercise it. Some people live to give away secrets.
