The short answer is constantly. The market is always evolving and your target audience is evolving along with it. Markets may seem stable, then transition occurs. You can't just define your target audience once and then forget about it in a desk drawer or deeply buried folder. As you interact with more customers and notice new challenges or problems, make sure you're keeping notes so you can adapt existing buyer personas or create new ones. This way, you’ll always be aware of new challenges and how to solve them.