You can see good coverage on your specific question, but how is you 4/5 week length working for your team, client, system?

Have you tried other lengths? Did they deliver better results, was it easier to plan, deliver value, was the team happier?

How is your overall implementation of agile going? Is the 4/5 week time box helping you adopt the other part of the agile/scrum receipt for software development?

Have you stopped improving? Where did you start with your sprint length? Are you and your team actively managing the duration down like Ken Clyne talked about?

Based on those answers, I would say that you need to actively manage it yourself and skip the rules of thumb.

You can see good coverage on your specific question, but how is you 4/5 week length working for your team, client, system?

Have you tried other lengths? Did they deliver better results, was it easier to plan, deliver value, was the team happier?

How is your overall implementation of agile going? Is the 4/5 week time box helping you adopt the other part of the agile/scrum receipt for software development?

Have you stopped improving? Where did you start with your sprint length? Are you and your team actively managing the duration down like Ken Clyne talked about?

Based on those answers, I would say that you need to actively manage it yourself and skip the rules of thumb.