The comparison to the Avatar timeline isn't very good. Avatar was announced before any design work has been done. The design work takes at least 2 years. I can tell you right now they were working on design work as far back as 2013, which already puts it 2 years ahead of the avatar announcement.
I would be very surprised if the expansion is NOT completely done for the 50th anniversary, so I would expect it to wrap up sometime in 2020, which is still 5 years from now. I know it's not impossible for Disney to drag these things out, but my take is they will want to push this a little faster seeing as its Star Wars.