A new British survey asked 2,000 people how long the 'no spoilers' rule should last . . . and it actually produced a definitive answer.
According to the results, you need to wait: 33 hours after a TV show airs and 10 days after a movie is released. Which is so oddly specific that it MUST be right. That makes sense though: Don't talk about a TV show episode the next day . . . wait for the day after that.
And don't drop spoilers about a movie until it's been in theaters for two full weekends.
The only tricky thing is: How does this apply to Netflix series like "Stranger Things", which drop an entire season at a time. The survey didn't cover that scenario, but off the top of my head, I'd suggest two weeks.