I suspect we'll see a new book from Jim every spring, and it'll alternate between a Dresden book and a Cinder Spires book, until the latter series is finished, then back to a 12-18 month cycle for the Dresden Files (until Jim starts a new series).
I don't know if it will be that bad. Look at the release dates of the Codex Alera books and the Dresden File books. You don't see an alternating pattern of one book a year. Also, Jim said that as soon as he finishes the Aeronauts Windless he'll start on the next Dresden book, which kind of surprised me, I thought he'd need to take some time off before starting book 16; or Peace Talks if that's what it will be called.
Perhaps because the delay in finishing the first Cinder Spires book was caused by dental problems and not some kind of writers block, which Jim has said he doesn't believe in, he already has a strong grasp of what he needs do to finish the first story off and it won't be a great leap for him to jump back into the Dresden Verse again. He also said it takes about six months to write a novel, not counting exploding teeth and other complications life can throw at you. So here's to Jim having few life complications to deal with so your prediction of either a Dresden or Cinder Spires book per year; but not both, doesn't occur.