When answering this operational question, you are required to make a judgment call. Since there is no final stage in which everyone will agree that a project planning exercise is complete, you should address when you believe it is complete and how you go about communicating this to the team. This question should not be that hard for you due to your experience as a business analyst.

For examp: "Determining when the project planning process is complete is challenging, as is confirming that all the requirements have been met. I have a set of criteria I use to confirm this and then communicate these to the team, both at the beginning of the project and once the planning phase is complete. These include aligning the requirements with the business objectives of the company, allowing all the stakeholders the opportunity to present their views and ideas, establishing clear benchmarks for each stage of the project, and finally, making sure the resources needed to complete the project are in place."