In Windows Phone 7[.x] background agents would expire (stop working/running) if an app wasn't launched for 14 days. Today I noticed in MSDN that things are slightly different in Windows Phone 8: 

There are a few cases where an app’s background agent will  not expire or will be automatically renewed. 
•  If the Tile for your app is pinned to the phone’s Start screen, the expiration time for your background agent  schedule is automatically extended to two weeks whenever your background agent calls Update(ShellTileData) to update the Tile. 
•   If your app is selected by the user to display notifications on the  on the lock screen, the expiration time for your background agent schedule is automatically extended to two weeks whenever your background agent calls Update(ShellTileData) to update the lock screen. 
•   If your app auto-uploads photos using a resource-intensive agent, the resource-intensive agent schedule will not expire as long as the user has enabled this feature for your app in the photos+camera settings page.