A push notification is a message which is "pushed" from Pangea CMS to user interface. There can be only one push notification item at the time either article or placeholder. And there is no duration setting.
Push Notification Settings
To navigate to Push Notification Settings go to Sections > Push Notification. Adjust search criteria if needed and hit Search button.
Once you find article you would like to push add it by green arrow. There is also option to Add Teaser and Add Placeholder which can be any customized message.