I went with "use you own discretion" as I usually err on the side of caution.
I think six months is fine and posting open spoilers in a thread clearly marked for spoilers for a certain topic is fine.
I put a post about the ending of "Synthesis" in spoiler code just yesterday. As I had just finished it I thought there might be others in the same boat. I did so because the thread wasn't about "Synthesis" and I wanted to play it safe for others sake.