It is discarded when the expiration time you specified in the payload elapses (assuming you are using one of the enhanced binary formats which have the expiry parameter).
If you specify a non negative expiration time, the notification will be stored in APNS servers until that time passes or it is delivered.
A UNIX epoch date expressed in seconds (UTC) that identifies when the notification is no longer valid and can be discarded. If this value is non-zero, APNs stores the notification tries to deliver the notification at least once. Specify zero to indicate that the notification expires immediately and that APNs should not store the notification at all.
