Generally Speaking: Don’t be a douchebag.
For TV Shows That Release A New Episode Every Week: 24-hour grace period; no putting the name of the character who died in headlines or in tweets. But you are allowed to use code names or hashtags or otherwise non-specific descriptors that refer to The Thing That Happened, like “The Red Wedding” or “Six Minute Tracking Shot” or “Tread Lightly.” After 24 hours, it’s open season.
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.
For TV Shows That Release All Their Episodes At Once, Like Netflix:
24-hour grace period for the first episode of any all-in-one season, followed by a one-week grace period for the first four episodes, two weeks for 5-8, and three weeks for the full season.
For TV Shows Based on Books or Comic Books: Readers can talk about everything EXCEPT for character deaths. Also, everyone can do their best not to be a douchebag.
For Movies In General
Anything that happens in the first half-hour in a movie is not a spoiler. Everything after the first half hour should be clearly marked with a SPOILER ALERT until two Mondays after their release. Movies that initially open in limited release get a one-month grace period from their opening.
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. But in the interest of not being a douchebag, one ought always to err on the side of not ruining the movie experience.
For Videogames: Same three-week grace period as the season finales of binge-released TV shows. Not that it matters much, since there don’t seem to be any videogames coming out anymore.
For Books, mainly YA franchises: 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.
And If You Live in California: Don’t go on any social media for three hours before your favorite show airs. Because we’re watching it on the East Coast, and we’ll probably talk about it. Get over yourself; you got all the good weather in this deal.
(More lengthy explanations for these rules are right over here.)