Your MIDlet should handle Notification ID as follows:
The Notification ID (NID) is a unique value per MIDlet and per device. It gives additional privacy to users of your MIDlet, since it allows your service to deliver notifications to your users without knowing anything more than the Notification ID.
You as developer need to make sure that your MIDlet sends the Notification ID to your service.Your service needs to bind the current NID to that user's service context. This requirement is of course relative to the nature of your service, but is rather obvious for social media, instant messaging and other personal services.
Your MIDlet has to take care of this to avoid situations where a wrong person gets notifications actually targeted to another user of the same phone (remember: NID is MIDlet and device specific value).
Whenever the end-user of your MIDet changes, the MIDlet should report to the service to remove the specific NID value is used by this specific user, and remove the reference to the previous user's context.
The Notification ID does not expire. This means that the Notification ID stays active even if the user stops using the service.