The length of a Sprint determines how quickly a Scrum Team can “inspect and adapt” to changing circumstances and learning. Scrum, as a tool for product development, sets an upper limit to the duration of a Sprint. In other words, Scrum sets a minimum for the frequency of the inspect and adapt cycle. This ensures that teams using Scrum get at least a certain minimum benefit. Scrum does not set a maximum frequency (minimum Sprint length). If a team has a five-week (or longer) Sprint, the benefits from Scrum rapidly drop off. In particular, you dramatically increase risks associated with short term planning, responding to change, team development, windows of business opportunity, and error detection. Having a cycle longer than four weeks is not Scrum and a team with such a cycle length should not claim to be using Scrum.
Note: some references say that the maximum Sprint length is 30 days or one month. This is considered essentially equivalent to a maximum length of 4 weeks. Please see our article about Factors for Choosing a Sprint Length for more specific and practical guidance about this topic.