From an etiquette perspective, I would say that for TV shows, once a show has aired in your country, then that's long enough to wait. For movies, if it's a big plot twist, a couple of weeks is good, for books, about a month. That gives people who really want to watch it fresh a reasonable period of time in which to see it.
I would say from a practical perspective, if it really, really matters that you watch a TV show 'fresh' then it's probably best to avoid Facebook for the couple of hours until it airs in your area, for those to the west. If you miss a show, or choose to record it and watch it later, then that's the price you pay for the convenience.
I'm speaking from the point of view of someone who has totally given up on the idea of consuming media in its unspoiled state, as I tend to get things months to years after it's original air date (a month to six months for new movies, for most English TV shows the soonest I can legally watch them is about 2 weeks after they come out on DVD).