Do you start off with "So, that new Arrow ..." or something similiar. Generally speaking, it is less about an actual time, and more about how pervasive it is in pop culture.

Let's take Game of Thrones, and its source series A Song of Ice anf Fire (I will try to be vague as possible). What is reasonable for me to talk about? Some of those books have been published for a good, long time, yet people are still being introduced to the show. Are show only spoilers acceptable in public discourse? And how long after the fact? Yes, I have had good discussions about a battle that is about to happen in Winds of Winter, and the implications of the some of the more in depth fan theories, one of which is literally the great ______ conspiracy, and another is all about get hype, and the word bowl. Multiple characters/plotlines are discussed on /r/asoiaf because of what happened in 2011 with A Dance with Dragons. but the subscriber number of /r/gameofthrones/ should tell you that there are more folks on reddit that watch the show then read the books. This is why I don't like the argument "It's your responsibility, it's been out in the open for x years" as then I can just openly talk about Lady SH in a bar, and inevitably get into a heated discussion on spoilers.

Or something like another show. Hannibal. It is not greatly watched compared to some shows, and it is very loosely based on a series of books, with a series of movies in the pop culture. But bloody hell, I do not talk about that show in public. Only with people, in private or on specific forums, where people are completely caught up on the show.

Yes things like Vader, Rosebud, Dumbledore's demise, The Sixth Sense, Keyser Soze, Tyler Durden, these are all effectively public domain to various degrees at this point. On the other hand, people are still deciding to watch Twin Peaks on Netflix. Or Buffy, or Angel, or Carnivale, or a slew of other shows that have ended. And others prefer to binge watch after it is available to stream/purchase a season. In the end, I personally try not to get into spoilers unless I already know the level of discourse I can engage in, and try to "warn" people with the show/book/whatever in the first line. People who avoid spoilers/discussions know well enough to stay away. Yes, when restaurants put up street menus lampshading a plot development on Game of Thrones, or gets thrown around loosely on sites like facebook, it gets tougher to defend that it come coupled with a spoiler warning. On the other hand, people get busy, or people just were not interested/paid attention/were old enough when the media first was published, so there is that. Then again, people even came up the machete order for the Star Wars films to indoctrinate their kids, so that the storytelling of Episodes IV-V are preserved, followed by the prequels, before wrapping it up with VI.