24-hour customer service window
Updated
Quick answer: The customer service window is the 24 hours after a customer's most recent message to your WhatsApp Business API number. During it you can send any free-form message — text, media, buttons or product lists — and Meta does not charge for those replies. After it closes, you can only contact the customer with an approved message template.
The period after a customer messages you when you can reply freely.
Each new customer message restarts the 24-hour timer. Conversations that start from a Click-to-WhatsApp ad get a longer 72-hour free entry point window.