24-Hour Messaging Window
Businesses and developers using the Send API have up to 24 hours to respond to a message sent by a person in Messenger when using standard messaging. A bot may also send one additional message after the 24-hour time limit has expired. The 24-hour limit is refreshed each time a person responds to a business through one of the eligible actions listed in Messenger Conversation Entry Points
. This is commonly referred to as the '24 + 1 policy'.