re: How much time has to pass before something is longer at risk for a spoiler?

Probably something like 10 years but even then there will be some outliers. For instance, someone here today said they had never seen the Godfather movies. But hey, after 10 years if you haven't seen a particular movie/TV show there's a good chance you won't and even if you got "spoiled" after so many years will you even remember the spoiler?