Somewhere between 6 months and just use your own discretion. As in, no code needed if you're in a thread that says it contains spoilers. As that's the point of being in that thread, you can talk openly about what happened there.
And obviously NEVER in a thread title, at least if it can be avoided. I guess if you, for example, want to ask a question about
, it could be considered spoiler-ish about the current state of that issue vice where the show left off. I guess 6 month rule works there, nothing in the open that's new, and avoid it as best as possible for thread titles.