Messenger keeps messages until they are manually deleted by the sender or the recipient. However, there are some exceptions to this rule. For example, if a message includes a video or photo from someone's camera roll, Messenger will delete it after it is viewed. Additionally, if a user deletes their account, their messages will be deleted as well. It's worth noting that Facebook may also save messages for a limited period of time to improve their systems and processes, but these messages are generally anonymous and not associated with specific accounts. Overall, Messenger users should assume that their messages will remain on the platform indefinitely unless they choose to delete them.