the formula is (( Y - X ) / P )+ R
x = length of time since book/movie/tv show came out
y = length of time until the next installment
p = ppopularity = how likely it is that the people youre talking to have seen/read it
r = relevance = how important the specific spoiler is to the story.
and while many people who spoil stuff are morons, i also believe that some of the responsibility falls on the people who dont want to be spoiled. If I dont see a movie, I dont read threads about it. I almost avoided the net completely between Friday night and Tuesday morning when I finished Harry Potter, just to be safe, because I KNOW that there is a higher likelihood of that getting spoiled by some yutz. (who put the spoilers in his sig elsewhere on ENWorld)
some examples:
vader is lukes father. The main reason this isn't a spoiler is because the sequel already came out. Once sequels come out for ANY mainstream movie, the prior movies are 100% safe to spoil without worry.
In geek circles, it is probably safe to assume that everyone interested will go see a geek-movie within a few weeks. Outside of geek circles, this isn't necessarily true.
comic books run monthly. BUT they are a lot less popular outside of comic circles. So here it depends on how relevant the spoiler is... if its just a result, its not necessarily a big deal. if its a swerve, don't give it away. (more on relevance below)
tv shows however run weekly. the next episode is often only 7 days later. Therfore the spoiler time is MUCH shorter. Talking about plot points of recurring shows should be pretty safe, of course it usually isnt necessary to do so without knowing if the people youre talking to has seen the episode. people who watch these shows religiously are responisble for 1> staying up to date or 2> letting everyone know that they arent up to date.
soap operas run daily. if someone missed yesterdays episode, they probably WANT you to spoil it for them so they can keep up.
the problem is that tivo, and tv-watching on DVD have confused things, but thats why responsibility shows up. When I first met the DM of one gaming group, he would constantly tell us that he has everything taped...and hasnt watched it yet untiil he knew that we knew not to spoil stuff without checking that he watched it. I do the same thing if I wind up behind.
Relevance is the most important thing.
The reason Song of Ice and Fire conversation is always hidden is partly because of popularity (many assume that others havent read it) but mainly because of relevance. The twists the story takes are a very big part of the reading experience.
A movie like Sixth Sense or the Others is centered around the twist and you will absolutely ruin the movie for anyone who hasn't seen it. The twists should never be discussed without clearance.
so yeah, you probably blew it with the Others. but you should have quickly tried to score points by saving them from watching that truly awful movie.
edited for extra parenthesis in equation