According to the guidance for effective quality management, when should the requirements for a product...

In PRINCE2, the requirements for a product should be captured early in the process, specifically when the project team realizes the need for that product. This approach ensures that the project delivers what is actually required and expected. Capturing requirements early helps in detailed planning and allows for effective quality management throughout the product's development and delivery phases.References: (PRINCE2® 7 Foundation Handbook, AXELOS)