Replying to Facebook messages older than 24 hours

Facebook enforces a strict 24-hour messaging window to protect users from spam and prevent businesses from sending unsolicited marketing messages, making it impossible to reply to older Facebook Direct Messages using standard automated tools.

Rergardless Facebook has defined an exception that messages can still be answered more than 24 hours after they have been received. 

Manual reply (Human-Agent)

Please note: This option is only available for messages that are a maximum of 7 days old. Older messages can only be replied to directly on Facebook.

This option allows a customer service employee to reply. With this option, it is possible to reply to a message up to 7 days after it has been received. Automated messages and any content that has nothing to do with the user's original request are prohibited.