The length of a Sprint should be:

According to the Scrum Guide1, the heart of Scrum is a Sprint, a time-box of one month or less during which a “Done”, useable, and potentially releasable product Increment is created. Sprints have consistent durations throughout a development effort. A new Sprint starts immediately after the conclusion of the previous Sprint. The length of a Sprint should be short enough to keep the business risk acceptable to the Product Owner, short enough to be able to synchronize the development work with other business events, and no more than one calendar month.

References: Scrum Guide