The Product Owner must spend enough time with the Developers so that they are confident the Increment will meet the intended value. This is because:
* The Developers are accountable for creating a "Done" Increment in every Sprint. They must ensure that every Product Backlog item they work on meets the Definition of Done before it is considered complete.
* The Product Owner is accountable for maximizing the value of the product resulting from the work of the Scrum Team. They must inspect the Increment at the end of each Sprint and assess how it delivers value and contributes to the Product Goal.
* The Product Owner and the Developers must collaborate closely throughout the Sprint to ensure that
* they have a shared understanding of what they are building and why. The Product Owner must provide clarifications, feedback, and guidance to the Developers as needed to help them create a valuable Increment.
* The amount of time that the Product Owner must spend with the Developers may vary depending on the context, complexity, and uncertainty of the product development. However, it should be enough so that both parties are confident that the Increment will meet the intended value.
Other options, such as 40%, as much time as the Developers tell them to be present, or 100%, are not valid answers as they imply a fixed or arbitrary amount of time that does not reflect the actual needs and dynamics of the product development.
References:
* [Scrum Guide], page 7, section "Developers"
* [Scrum Guide], page 6, section "Product Owner"
* [Scrum Guide], page 10, section "Definition of Done"
* [Scrum Guide], page 10, section "Increment"
* [Scrum Guide], page 10, section "Product Goal"