Well, now that Skin Game is finished, and has an official release date, I guess Jim Butcher will pick up working on the Cinder Spires books. I hope the first one is out sometime mid-to-late 2014.
He was probably around 2/3 done (estimate based off of the assumption that it has about as many chapters as SG, which is not necessarily valid) before he put it down to work on SG. Now that he is back at it he has decided to do some rework at about the half way point it seems.
He has said in the past that back when he was writing Codex Alera, switching between the books with such different characters and PoV styles (The CS books are 3rd person like CA) was quite rejuvenating to his creative process so it could also increase his productivity with the DF and vice versa.