Example: Your sprint duration is 4 weeks. So how long does it take to deliver an increment of business value (where that increment is doable in the course of a single sprint)?
- Best Case: 4 Weeks (request arrived just in time to be included in the next sprint)
- Worst Case: 8 Weeks (request has to wait out a full sprint before being prioritized)
- Average Case: 6 Weeks (1 1/2 * sprint length).
By mainting a constant 4 week sprint duration, you can guarantee to management or to customers an average response time of 6 weeks, and a worst case of 8 weeks. If the sprint duration is variable or not defined, it is very difficult to measure or calculate how agile the team is, which is probably a synonym for "not very".
BTW - there are a variety of strategies for improving this measure of agility: shorter sprints, multiple teams with overlappying start dates (you're never more than 2 weeks away from a sprint planning, so you average response time is reducted from 6 weeks to 5 weeks, and the worst case from 8 to 6 weeks).