Final answer:
The best time to implement new customer expectations into the Sprint Backlog is during the next Sprint Planning.
Step-by-step explanation:
When new expectations are detected from the customer, a good time to implement them into the Sprint Backlog would be in the next Sprint Planning. This allows the Product Owner to properly evaluate and prioritize the new expectations alongside existing items in the backlog. Implementing them right after they are detected may disrupt the current sprint, whereas waiting until after the sprint or before the next sprint may cause delays in addressing customer needs.