I think it also depends on context. I agree with saying a few weeks... for general internet discussion/tumblr/whatever. But OTOH, I was reading a book circa the 1970s and was really into it, and then my mom came over and casually spoiled the ending for me. That was just rude, even if the book has been out for decades. I think it's rude to knowingly spoil something for someone no matter how long it's been.

ETA: And also, I think it somewhat depends on the spoiler. There are some things I wouldn't want a Breaking Bad newbie to know about because the surprise is super worth it. I always tell people to watch The Usual Suspects before they get spoiled for it because the "twist" is so fantastic unspoiled.