How well is your Product Backlog maintained?
The Product Backlog is an ordered list of everything that is known to be needed in the product. It is an art and technique in itself to master the shape and content of the backlog items, let alone create a metric that would tell if the Product Backlog is in good health.
Product backlog in good shape has the following characteristics: it is detailed appropriately, emergent, estimated, and prioritized. These can be summarized with the acronym DEEP.
Product Backlog items that can be "Done" by the Development Team within one Sprint are deemed "Ready" for selection in Sprint Planning. For a Product Backlog Item to be in a state called "Ready", several conditions must be satisfied. First, make sure that the item is Independent, Negotiable, Valuable to users and customers, Estimable, Small and Testable (INVEST). Second, make sure the Product Backlog Items have Acceptance Criteria (or Definition of Done) written with a consensus of a team prior to estimation. Estimation is usually done using story points or ideal days as a measurement unit and a very frequently used estimation technique is Planning Poker. Third, a few sprints must be executed for a Team Velocity to become relatively stable.
For a Product Backlog to be in good health it must contain enough User Stories in the "Ready" state with a cumulative number of Story Points greater than average Velocity. If the total number of Story Points of User Stories in "Ready" state is about 2 or 3 times the average Team Velocity, then the Product Backlog is in even better health. Having more User Stories in "Ready" state than that is wrong since it indicates too much planning upfront and negates the emergent nature of the product backlog.
The health of the product backlog ensures to maximize the value resulting from the work of the development team. Assuming you are not satisfied with the current state of your product backlog, you would like to improve on how the product backlog items are composed, how many of them are in such a state that they can be pulled into a sprint backlog and start working on them immediately.