A fixed iteration length enables a team to establish a development and delivery rhythm. It facilitates improved estimation accuracy allowing the team to obtain a tangible feel for what they can deliver within an iteration. Rhythm is an important factor, which helps a team achieve a sustained pace. In a 2-week iteration, the team get used to planning for 0.5 days at the start of an iteration, developing for 9 days with increasing intensity, and then reviewing the functionality with the customer for 0.5 days at the end of an iteration. This cycle provides a consistent environment for the team.
I’ve worked in organisations where iteration lengths were varied depending on the mandatory functionality selected for an iteration. It’s not possible to establish a rhythm when the length of iterations is varying. An established rhythm drives the team, subconsciously, along a fixed timeline enabling them to concentrate on developing the functionality. A varying iteration length is simply another variable to think about and administer - and this introduces some risk and management overhead. Ideally you want to keep as many variables as possible constant.
