With one exception, BL has a simple and overarching rule: if it isn't
comprehensively translated into English yet (officially or unofficially), use spoiler tags. This means that if something's only been summarised - or had a few sentences translated - it's still verboten for open discussion. But as, for example, each English Fate/Apocrypha chapter is released, you can start talking about it happily. (For Fate/EXTRA CCC, this means you should use spoiler tags for everything not covered by the English Let's Play and a few scenes filled in by the translation forum: notably, the CCC route's unique events are therefore still under embargo).
The exception is Fate/Grand Order, which has its own spoiler policy,
as detailed here.
For
non-Type Moon works, we have no particular policy beyond requiring you to have some consideration for other members of the forum: we will frown upon spoiling the end of a long book/game/whatever the day of its release, for example.
Occasionally, we also manually designate certain threads as either spoiler zones or spoiler-free zones, in the title or OP. Translation threads often end up being spoiler zones where information from throughout the work is likely to be given away without tags; conversely, in some threads we place an embargo where spoilers would otherwise be allowed (Prisma Ilya's anime thread is meant to remain free from manga spoilers, for example). We will make these exceptions clear, so just do your due diligence and spend ten seconds looking at the thread title and OP before you post, and you should be fine.