TTL (time to live) – a value interval (in seconds) that specifies how long Viber should try to deliver the message.
TTL value ranges from 30 to 1209600 seconds (14 days). If no TTL value is set, Viber will try to deliver the message for up to 14 days. You may choose preferred TTL by yourself. Messages delivered after TTL has expired, won`t be charged.