I guess spoilers are like beauty or being offended and is in the eye of the beholder, what I may find beautiful, inoffensive or unspoilery, others will find ugly, offensive or spoilery.
So what to you is a spoiler, when do you think the line changes between a perfectly fine plot detail that can be openly discussed and the point of discussing it, would ruin the enjoyment of yourself?
Does the magical line occur for you? A few days after the release of the book, a month, six months, a year, a decade or two or should say, something like Darth Vader being Lukes father in Star Wars be always considered a spoiler even though that occurred before I was born?
My personal opinion is, the longer out from an event, the less spoilery I consider it, say the events of The Eternal Tide or last years DRGIII Typhon Pact duology I wouldn't as they were out a year ago, but say books like Birth of the Federation, I would be more cautious in discussing without considering spoiler tags.