"The 'Burn Notice' is a Silent Phone feature that allows users to decide how long a message is available before it is deleted from both the sending and the receiving device.
The 'Burn Notice' feature is always 'On' by default and can not be turned off. You can, however, determine how long messages are available: between 1 minute and 90 days (the default setting is 3 days).
During an active conversation, tap on the fire icon to the left of the composing window and use the slider to determine how long your messages will be available.
Once you have sent a message and when the timer expires your message will be deleted from both your device and the receiving device. You can view the ‘Burn Time Stamp’ under the flame icon attached to each message.