Originally, GL himself said the timespan was three years between each movie. Then, when Secrets of the Empire came out, the time between Empire & Jedi was changed to one year.
That always bothered me, as Yoda told Luke he had to stay with him to complete his training. Then Luke went off on his own and was able to do 25 years worth of Jedi training (Obi-Wan was knighted as a Jedi Knight at 25) in one year all by himself. He also tells Luke that only a fully trained Jedi can face off with Vader, then tells him in Jedi he has to confront Vader to be a Jedi Knight. I know it’s all because when Empire came out the plan was to do 6 or 9 movies, and then Lucas had to wrap it all up in one move.
There’s a great video on Youtube where a guy tracks down all the old articles about Star Wars being a 9 or 12 movie run. There are quotes with Lucas and Gary Kurtz talking about Luke’s sister being a completely different person on the other side of the galaxy. She was to get her own trilogy and then Luke and his newfound sister would team up to fight the Emperor. There was also talk of Luke killing Vader and taking his place, and his sister would bring him back to the light side. It’s a very interesting video if you can track it down.