Around the time The Empire Strikes Back came out—and the first Star Wars film was retitled Episode IV: A New Hope—there were a number of apparently authoritative statements that the Star Wars series was eventually planned to to have nine film installments. (For example, that claim appeared in the interior notes for the original two-LP soundtrack for Empire.)
By the time there was serious talk of the prequel trilogy being made, I recall Lucas saying that he did not intend to make more than six Star Wars movies (and maybe that he had never intended more than six). It seems now that he never had any fixed story ideas for after Return of the Jedi, but there was certainly a time in the early 1980s when a nonologynonalogy was the announced goal of the Star Wars series. So when was it actually first indicated that Lucasfilm did not have any plans for more than six?