User blog comment:Salubri/Aizen, Yamamoto and Wonderweiss/@comment-1454424-20100227023803

Only thing Kubo said about the future arcs is that Isshin will play a bigger role in the upcoming arcs. Doesn't mean it will focus on him and the Turn Back the Pendulum stories were just 1.5 volumes. I don't think Kubo considers them separate in any way from the Arrancar Arc. My guess is Kubo's definition of a short arc is at least 4 volumes, i.e. roughly one year. So that's 40 chapters. Long enough to have a Isshin gaiden, Kenpachi gaiden and more!