There are three scenarios you may encounter when the policy is implemented. 

1. Chat messages that are 30 days or older: 

These messages will be immediately deleted following Microsoft’s scheduled deletion job schedule. 

2. Chat messages that are less than 30 days old: 

Any chat message less than 30 days old and X days old will remain visible for 30 – X days until it is deleted. For example, if a message is five days old when the policy is implemented, the message will remain visible for another 25 days. 

3. Net new messages created after the policy is implemented on September 16: 

Any message created after the policy is implemented will have 30 days until it is deleted. 

Please note that Teams Chat deleted by a retention policy is gone permanently and cannot be recovered. Everyone across the tenant will be subject to the retention policy.