How long is an iteration?

This is a question I have been presented with often. If I suggest two weeks there is usually someone around who can make reference to a SCRUM text book that states 4 weeks. I’ve heard good arguments for three week iterations, often along the lines of a compromise and that you are left with a clear week in the middle to be ‘in the zone’ from a development perspective.

Recently I’ve had a team I was coaching ask to be allowed to make iterations longer. On occasion I’ve challenged teams to consider shortening iterations.

But what iteration length is correct? A short iteration serves to expose the inefficiencies that can be hidden otherwise. 

For example; a team cannot work on a two week iteration because of the time taken to prep for the demo. What do we do? We can extend the iteration to 3 or 4 weeks at which point we can stomach the inefficiency or we tackle it head on identifying where the pain is emanating from.

On the other hand if a team running a three or four week iteration is comfortable do we have the courage to challenge that. By reducing the length of the iteration we increase our feedback rate and also give ourselves the opportunity to identifying the next hidden constraint that is limiting our throughput.

In short there is no right answer. I tend to lean towards two weeks, four weeks feels too long to me for most teams particularly where we need to learn and want that rapid feedback.

What are your experiences?