I suppose it's best to take it on a case by case basis in the context of where you're talking.

The original Star Wars trilogy is so entrenched in the cultural zeitgeist that people who haven't even seen it are aware of the basic plot points and the "I am your father" line.

Harry Potter is such a huge phenomenon that the people who care about Snape's true nature would have been invested in it and found out about it already. If for whatever reason you found someone who just now got into Harry Potter, they're going to have a hard time avoiding those landmines anyway.

If I'm watching a new episode of The Walking Dead, I won't have a conversation about it on my Facebook wall. I'll reserve it to private messages and in The Lounge thread. But i'll say this: If we're on season 8 of a show, I think the first 7 seasons are fair game for spoilers. For the people who just started watching it, see the Harry Potter example.

As a general rule, when something new comes out be aware of your surroundings when discussing it. I'll leave it to other people to decide when the cut off point for new is.

If i'm a little late to watch something, like i'm at work when it airs on television, i'll avoid social media until I watch it. Some people say that's not fair, but if you like talking about shows with your friends, it might just be the nature of your feed. I've found that people are a little more courteous about posting movie spoilers in their feed, because it takes more effort to go see those as opposed to a show lined up on the DVR.