At the heart of the general concept of agility is the freedom and flexibility to improvise and innovate. This sense of freedom should apply as equally to the output/deliverable/outcome of the project at hand as it does to the processes that are employed to get the work done.
Within the broad framework of agile marketing methodology there is plenty of room for practitioners to find their own way through the same sort of experimentation that the underlying philosophy of agile methods encourages and promotes. How long should a sprint be? It depends on the project. How often should teams SCRUM? Sometimes meeting on a daily basis is just too often and three stand-up meetings a week are more appropriate. Do what works best for you!
With agile marketing the rules of the game are such that changing the rules to suit you and your team are well within bounds. This notion of a fast, flexible and methodical, yet malleable, process can help marketers to break out of the confining strictures of hierarchy that often hinder the employment of agile methods and the work that they aim to produce.