Early 2000's. I heard about this in early 2000's and found info about it on Wikipedia. In short he stated that Star Wars was his and that he'd be too old to direct more trilogies and didn't want anyone else making Star Wars movies:
In an interview published in the February 1999 issue of Vanity Fair, Lucas said: "When you see it in six parts, you'll understand. It really ends at part six. I never had a story for the sequels, for the later ones." In early May 2002, just before the worldwide release of Episode II: Attack of the Clones and while Lucas was working on the script for Episode III, rumors of Star Wars episodes VII, VIII, and IX were posted on the Internet. In reply, Lucas noted that there would be no such sequels, since the time felt right to move on:
I am going to do my own movies. I have got some ideas but they are the kind of movies that aren't going to be popular, they're not going to be successful in terms of financing. I have managed to get a fund by doing these (blockbuster) movies that allows me the creative freedom to do things that may never see the light of day—or if they do get distributed they will be on a very limited basis because they are not mass entertainment movies. There are a lot of things I still want to do. I want to do a TV show, some TV movies, mostly they will be historical in nature.
In 1999, when asked about the possibility of someone else making Star Wars films, Lucas said, "Probably not, it's my thing." In August 1999, at a press conference in New York City to discuss The Phantom Menace, Lucas described the "nine year commitment" required to make a Star Wars trilogy. In 2002, he said: "Basically what I said as a joke was, 'Maybe when Harrison and Carrie are in their 70s, we'll come back and do another version.' The thing I didn't realize then, and that I do realize now very clearly, is that not only would they be in their 70s, but I would be in my 70s too." He also noted, "Ultimately, the saga will be six films, a 12-hour story. Then people can watch all six films together as they were intended to be seen."