Spoilers

What is the statute of limitations on spoilers?

(Warning, may contain some spoilers, depending on how they are defined)

When is it okay to discuss the ending of a book, movie or TV show? One might argue, you can never discuss it with someone if they haven't seen it, otherwise you've spoiled it. That's a good point, but what about media that is well-known? Can I discuss Darth Vader's relationship with Luke Skywalker and not worry about spoilers? What about the endings of "The Wizard of Oz" or "Rocky?" At what point do these pieces go from unknown to common knowledge? Certainly I can't talk about the ending to the newest Harry Potter book, but what about the ending to the first book?

With books, I feel that one indication that the book can be discussed freely is if the book has been turned into a movie. But still, is it okay to question the fate of Frodo and Sam in the "Lord of the Rings" or is that a spoiler?

A good friend of mine once spoiled "The Usual Suspects" but it was okay because I saw the Academy Awards before I saw the film, and the awards indirectly spoiled the movie anyways.

So what are good milestones to look for when trying to determine if something is spoiled?

The inspiration for this post is the third Harry Potter movie, which I saw last night. There's something about it that really bothers me, but I can't discuss it here because it really spoils the movie.