Honestly, I don't think spoilers necessarily completely ruin things for me, but I will lose all immersion knowing they're there. Like, if someone says "Movie X was pretty good, but I hate those plot twists where you find out the main character was just dreaming. I wish they didn't have that for an ending," Movie X can still be freaking amazing, but I'll spend the whole time trying to figure out how that dream ending fits in with everything. It won't have the same effect.
As far as time until things are spoiled? I was reading a random thing on Mass Effect 3. Since I haven't played that yet, it was nice when spoilers were hidden, so I could avoid them. So, personally, unless it's a classic that's being spoiled by an English teacher somewhere, I'd rather light warning ("Everyone's seen Movie X, right?" is kinda nice, and should give away what you're about to say) first. Of course, if you're just eavesdropping on someone's conversation, and they spoil the movie for you, try focusing on some other noise if you don't want it spoiled. Random people in Starbucks should be able to chat about Movie X the second they watch it (not sure what else Starbucks is good for, anyway) and not have to worry about spoiling it for someone who isn't part of the conversation (plus, if you don't know they're talking about Movie X, it may not be about Movie X).