I've added a spoiler warning to the post, but you can't unring a bell, so I apologize. Over the years I have wondered what is the mandatory spoiler warning time for a movie or TV show. In real life, I frequently ask people if it is okay or not to reveal a storyline when we are discussing something. With the situation reversed, I usually wave off spoiler rights unless I have concrete imminent plans to see the work in question.
With box sets and DVRs, the window of opportunity to see something is very elastic. I frequently DVR stuff and watch it in batches. I have the entire season so far of Heroes recorded and I may never get to it. Every commercial for the show includes elements that would count as a spoiler to me, but that isn't NBC's fault. It would be unreasonable for me to expect the entire world to stay silent because I'm too busy to watch it.
Spoilers are a long and venerable tradition. National Lampoon used to carry a column called "Spoilers" that had one sentence summaries of the endings of movies in current release. I have no idea how many movies have been "spoiled" for me by Mad Magazine. You can't parody something without revealing the plot. Stephen King is quoted in Wikipedia as saying there is no such thing as a spoiler, the work should stand or fail even if you know the ending. Still, some sensitivity should be exercised and a certain blackout period should be honored.
My worst fail in this regard was on the opening day of Star Trek II: The One Where Spock Dies. It had been widely reported in the general press that Spock died and how controversial that was. The newspaper review that morning had let on that Spock died. As we were walking out of the theater there was a huge line of people there to see the next show. A complete stranger asked me how it was. I shrugged my shoulders and said, "Spock dies." People all around us sighed crestfallen. I had done a Very Bad Thing by not recognizing that others weren't as media-aware as I was and didn't know that Spock dies in the movie they were about to see.
Ooops! The previous paragraph should have had a spoiler warning. Otherwise people might learn that Spock died.
My personal rule of thumb for spoilers is one month for movies and one week for television shows but even that is a little flexible. The bigger the surprise the longer it should be kept secret. In reverse, the more popular the event, the less caution is required. Almost by definition, the result of a massive cliffhanger can't be considered a spoiler after it has aired. Who wasn't discussing Who Shot JR the very next day?
Since opinions vary, I have started a poll to see if I am alone in this stance.
Now that that is over, read no further if you don't want have other items spoiled for you.
Spoilers start here:
The final cylon is Tom Zarek's brother Muffit, played by Dirk Benedict. The kid is really a ghost and Bruce Willis was the one who killed him. Rosebud was Charles Foster Kane's nickname for his penis. Obi-Wan Kenobi is really Luke's dad. There are a few things Queen Padmé Amidala hid from Anakin. The island disappeared because Gilligan turned the wheel on the hatch after the Skipper told him not to.