Notifications will show as long as the pre-defined value is exceeded.
E.g. you have configured a notification for a temperature above 25°C.
At 1pm your sensor exceeds the value - a new notification is opened. You can see how long the notification has been active.
At 5pm your temperature drops below 25°C again - the notification is closed. You will be able to see that the notification was active for 4 hours.