According to Entertainment Weekly:
For TV Shows That Release A New Episode Every Week
The 24-hour grace period still suffices for the classical episode-a-week model. No putting the name of the character who died in headlines or in tweets.
For TV Shows That Aired Months Ago in Britain
If you are the kind of person who watches the shows when they first air – presumably totally legally, when you’re visiting your British cousin specifically just to watch British television – then it is incumbent on you to presage anything you say about unaired episodes with “Well, I’ve already seen the season, so-” at which point everyone you’re talking to will cut you off. However, it is also incumbent upon American viewers to use the internet defensively. Don’t go to the Sherlock wikipedia page. Don’t read any British websites. Don’t speak to any English people at all.
For TV Shows That Release All Their Episodes At Once, Like Netflix
Everyone binge-watches at their own pace. But let’s be honest: If you were any kind of House of Cards fan, you probably at least saw the first episode within 24 hours of the show’s release. Hence, I would propose that we maintain the 24-hour grace period for the first episode of any all-in-one season.
For Movies In General
Anything that happens in the first half-hour in a movie is not a spoiler. For major movie releases, everything after the first half hour should be clearly marked with a SPOILER ALERT until two Mondays after their release. That gives moviegoers two weekends to see the films.
For Movies Based On Things
Movie adaptations are actually less problematic than their TV counterparts, since lately there’s been a rather exciting trend of films departing wildly from the source material. (See: The Mandarin in Iron Man 3.) But speaking as someone who has read pretty much every comic that is being turned into a movie, I think one ought always to err on the side of not ruining the movie experience.
A two-week SPOILER ALERT grace period should be granted, since some readers have to go to school and do their homework, while other readers have to go to work and suffer from the crushing certainty that they should probably be reading a grown-up book.
While according to Swoosh:
MOVIES: 2 weeks. That’s it. Two weeks is plenty of time to make plans to get your butt to a movie theater. I’ve heard some people say you should wait til it’s on DVD. That’s insane. If you’re waiting for the DVD then you don’t really care about seeing the movie.
TV SHOWS: Spoilers are fair game the next morning for television shows. If you really cared so much about a show that you can’t have it spoiled then you should have watched it with everyone else.
COMICS: You can spoil comic book story lines after about a week. True comics fans are die hard and they stay up on their story lines and no amount of spoiling is going to deter them from reading the new book in one of their favorite series.
BOOKS: You should never spoil a book for someone. It’s so insanely hard to get anyone to read anything. Why would you try to discourage someone from picking up a book?
VIDEO GAMES: Video games have the longest wait for any of the spoilers. That’s because games take a while to beat. You should wait about six months before you start tossing around spoilers.
CARS: Don’t ever put a giant spoiler on your car. Your Honda is never going to drive fast enough to make it fly through the air.