I'm trying to understand the meaning of this phrase in the Scrum Guide (especially the boldface portion):
Sprints have consistent durations throughout a development effort.
Specifically, what I am wondering is: what is the maximum frequency at which a team can modify its sprint durations, and yet still be doing "Scrum by the Scrum Guide"? In other words, what period is referred to by "a development effort"? Is it through the life of a product, a release, a totally subjective time-frame that the team determines?
(NOTE: To be clear, I'm not asking for rules / best practices around selecting Sprint duration. I understand that there are many reasons to avoid changing sprint duration, and also various rationales and recommendations around shorter vs. longer Sprints.)
"Throughout a development effort" refers to the time taken until the Product Backlog is exhausted, or the Product Owner determines there would be insufficient value in continuing with further Sprints. Hence it is the life of the product.
If Sprint duration is amended by a team, for whatever reason, then they will need to re-establish consistency for the remainder of the development effort. Failure to do so will compromise the efficacy of empirical process control.