Note: There's no such thing as a FixedLengthIteration, singular... [Eh? You've used the singular in the paragraph below.]
Tom and MaryPoppiendeck? recommend starting with FixedLengthIterations, and they go so far as to say that it is the starting point that "always works." The reason is simple: if you install a two week fixed length iteration into a shop that's been used to lollygagging around for six months between starting requirements analysis and deploying the product, *everything* is going to have to change to accommodate it.
-- JohnRoth
Compare: CatJuggling