ST Lucas was definitely planning this as far back as ROTS's release.

I felt like clearing this up since it seemed like a point of debate among some users. However...

This is NOT one of those "my uncle knows a guy" kind of BS things, but my dad once worked somewhere in NYC that had a lot of connections with Fox. He got to know people in the company. He was even invited to a press screening of Revenge of the Sith and he got a lot of freebies out of that which were passed onto me, including the Making of ROTS book.

Point is, soon after that, I guess Fox was eager to get plans into motion about the sequel trilogy, and I remember him coming home one day and saying "its gonna happen". It was probably just preliminary distribution discussions before the whole Disney blowout and Lucas obviously didn't solidify anything with Fox, but he was definitely preparing for it nine years ago.

 

I think Lucas, being of a business mind, probably knew years ago that there was an "end game" for selling his company involving future Star Wars films. But, I don't think he had any direct plans of actually making any of them.

 

Lucas has been quoted as far back as 1978 that the Saga was a twelve-part series. Then it wasn't. Then, after ROTJ, he said that the story was done at EP VI. He decided to sell Lucasfillm last year, and evidently one of the conditions of the sale was that the ST be made according to his outlines. In other words, there's no definitive answer...nor does it really matter.
However it came to be, we're getting the ST when no one had any reason to think we ever would.

 

I bet even Lucas had doubts at one time, but was always prepared for a "what if" scenario. Plagueis fans like to pin their hopes on the opera scene.

 

From clips of interviews of Hamill from back then. Yes,it clear he had this planed out for a while.

 

"I've left pretty explicit instructions for there not to be any more features. There will definitely be no Episodes VII-IX. That's because there isn't any story. I mean, I never thought of anything. And now there have been novels about the events after Episode VI, which isn't at all what I would have done with it. The Star Wars story is really the tragedy of Darth Vader. That is the story. Once Vader dies, he doesn't come back to life, the Emperor doesn't get cloned and Luke doesn't get married..."
Lucas, 2008.

"But there's no story past Episode VI, there's just no story. It's a story about Anakin Skywalker and once Anakin Skywalker dies, that's kind of the end of the story."
Lucas, 2008.