Messages are never deleted from the system

However, messages have a default expiry time of 90 days (shown as 2160 hours) after which they disappear from the App.

The remaining time until expiration is shown in brackets next to the message.

The App does not show the expiry time.

You can set a shorter Expiration Time for a specific message

Click on the Settings icon next to a message in Inbox or Sent.

Set the Expiration Time in hours by clicking on the up/down arrows rather than typing a number.

If you set a Expiration Time of 0 the message will disappear "immediately" from the App.