WhatsApp Session messages are any messages sent and received in response to a user-initiated message to your business. A messaging session starts when a user sends your business a message and lasts for 24 hours in the most recently received message. You can wait for users to message to your app, or you can send a Template Message to invite the user to respond. Regular text messages or media messages (i.e., any non-template messages) can only be delivered in this 24-hour active window.