According to this text, how long did it take to film The Return of the Jedi?  

"Filming began on January 11, 1982, and lasted through May 20, 1982, a schedule six weeks shorter than The Empire Strikes Back. Kazanjian's schedule pushed shooting as early as possible in order to give Industrial Light & Magic (ILM) as much time as possible to work on effects, and left some crew members dubious of their ability to be fully prepared for the shoot.[30]"
According to the provided text, it took a little over eighteen weeks to film Return of the Jedi. Filming started on January 11, 1982, and ended on May 20, 1982.