In this example, Windows warns users of low battery power, but there is still plenty of time before they have take action.
Show when: When an event occurs and the user is active, or a condition continues to exist.
If resulting from a problem, remove currently displayed notifications immediately once the problem is resolved.
As with action notifications, notify users of successful system events only if users are likely to be waiting for the event, or after recent failures.
Show how often: Once when the event first occurs.
If this results from a problem that users need to solve, redisplay once a day.
Annoyance factor: Low, as long as the notification isn't displayed too often.
Alternatives: If users must eventually resolve a problem, use progressive escalation by ultimately displaying a modal dialog box when resolution becomes mandatory.