The answer may be: sooner than you think.
The Chicago Tribune's Maureen Ryan reports that work is progressing on the series, with writers working on scripts for the proposed first season.
... Lucas and executive producer Rick McCallum want all the scripts for the show's first season to be completed before production begins.
"They are working with writers on scripts," a Lucasfilm representative said in response to a query. "They won't set a production date [for Season 1] until the scripts are done."
But work on the unnamed show's scripts appears to be at an advanced stage—which isn't surprising, considering the writing of the live-action show began two years ago.
The show would be set between the eras of Star Wars: Episode III—Revenge of the Sith and Star Wars: Episode IV—A New Hope, will not feature any major characters from the film series and is rumored to be in preproduction in Australia, though none of that is officially confirmed.
The show is being developed independently by Lucasfilm with writers from all over the world and has not found a home on any TV network yet.
As for casting?
"They met with people in several countries (including Australia) early on but they are not currently casting. They are not going to start casting until the scripts are completed," according to the Lucasfilm rep.