In answer to the original question, it was the early 1990s.

I actually did a systematic study of reviews of the trilogy, including retospective ones written in the years since their release, and it seems like 1987-1993 is when the tide turned. I attribute this to a few factors. One, Empire had backlash against it because it wasn't as fun or mainstream as Star Wars, and the reasons people liked Star Wars--being a clever commentry on pop culture, and being fun and humourous--weren't present in Empire. Two, the film needed to find an audience, and while everyday viewers saw the film once and found it decent, Star Wars fans kept coming back again over the years. Three, part of the reason they kept coming back was that the film is far better acted, written and directed, but not as spectacular--but on your home television in the late 80s and early 90s, things like acting and writing become more important than exploding Death Stars, which no longer impress. And finally, the original audience of Star Wars had grown up and was now in their 20s and 30s--and also in control of the media--and appreciated the more adult themes that might have bored them as kids.

So, by the early 1990s, suddenly Empire Strikes Back is a cult film, and Star Wars is the fun, mainstream success that isn't as cool to people who are really serious about movies and the series. I guess you could say it's a bit of a hipster thing, but with good reason.

You can see my analysis of the release reaction here: http://secrethistoryofstarwars.com/originaltrilogyreception2.html