When is it appropriate to post spoilers?

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Since The Last Jedi came out, I was wondering when it’s ok to post spoilers for movies on social media in general?

I know the Facebook group Star Wars Sithposting will have non-spoiler TLJ content, then spoilers will be allowed to be posted I think at the end of January.

Is a month after a movie is released a good time frame? Should it be shorter or longer?