The concept of a spoiler for a TV show is a product of the Internet/DVR era for a couple of reasons. Before these things existed, the odds that you would A) care if someone told you what happened on last night's Mad About You and B) have access to information about Paul and Jamie's latest comic misadventure before you watched the episode were pretty slim.
Those of us who live our lives on the Internet, though, know that the risks of being spoiled now are legit. If you want to avoid learning what happened on Game of Thrones 10 minutes ago, you'd better embargo Twitter and Facebook, and stay away from headlines on most media outlets, too. (Heaven help you if you are waiting to find out the outcome of a sporting event.)
So the problem of spoilers is clear, but the question is: Does anyone care? Browsing Twitter or Facebook the day after [spoiler] died on The Walking Dead is a great way to have your experience ruined, but is it just a few bad actors stepping up to make watching it less fun for latecomers, or are we inconsiderate as a culture? Thankfully, a survey of DVR users from TiVo can tell us how people really feel about spoilers.
The first thing to know is that most people are aware of them, and have had their entertainment fun capsized by someone with an itchy Twitter finger—78% of respondents say that they've had movies/TV/sports spoiled. The most common spoilers—major plot points on a TV show (64%) and character deaths on TV (56%)—aren't the worst, though. 23% of respondents say that the worst spoilers are the final score of a sports game. In other words, "OMG, I can't believe they killed Doyle on Angel!" might be the experience most people have had (we're trying to keep our spoilers well outside the spoiler grace period here), but "FAVRE THAT INTERCEPTION JUST COST US THE CHANCE TO GO TO THE SUPER BOWL" after the 2007 (or 2009, take your pick) NFC Championship Game before you've seen the game play out is the most devastating. It can still be fun to watch how Doyle dies, but the magic of sports is in the not-knowing.
So how soon constitutes a true spoiler? The people who responded to the survey are divided on that. Deep analysis of the survey from Deadline.com indicates that more than half of your friends will happily spoil something less than a day after a show airs. (12.6% of respondents say that after the show airs, it's fair to talk about it; 16.6% don't know what a spoiler is; 22.5% would wait less than a day.) Of the rest of us, "more than two days" garners a reassuring 22.7% of support, so not everybody is a total asshole. Also, Snape kills Dumbledore. Darth Vader is Luke's father. Bruce Willis was a ghost the whole time. Brad Pitt and Edward Norton? Same dude. The planet of the apes the astronauts land on? Totally Earth.