Explanation
If project requirements are generally stable and major changes are not expected in the near future, the Length of a Sprint may be set to be longer, 4 to 6 weeks. This provides stability to the Scrum Team members to work on the Prioritized Product Backlogrequirements for lengthy periods of time without having to go through the Create User Stories, Approve, Estimate and Commit User Stories, Create Tasks, Estimate Task, and other related processes that are conducted for every Sprint. However, if project requirements are not very well defined or if significant changes are expected in the immediate future, the Length of Sprint may be relatively shorter, 1 to 3 weeks. This provides stability to the Scrum Team members to work on shorter Sprints and deliver results, which can be evaluated by the Product Owner and stakeholders at the end of the Sprint. This also provides enough flexibility for them to clarify requirements and make changes to the Prioritized Product Backlog at the end of each Sprint.

Reference:
phases.html