With most long-running anime (Naruto and Bleach in particular), openings generally change every 25-26 episodes, and during the span of one opening theme, there are two ending themes, which run for 12-13 episodes. Now the current opening debuted on episode 231, so taking the above into account, we should have a new opening by episode 257, roughly in 8 weeks time.
Hope that helped.

@Konan-chan: You have to keep in mind that a weekly chapter is anywhere between 15-19 pages long, and depending what the contents of the chapter are, you can determine how fast the anime will get through the scenes. On average one episode will cover two chapters, so a span of 50 chapters can be covered within roughly 25-27 episodes (again, depending on the content, along with whatever filler scenes the anime team decide to add). So when you think about it, filler arcs are used so that by the time the anime gets back onto the canon storyline, Kishi will have a considerable lead on them, and then we can enjoy a long run of canon episodes without the story being interupted.
