Questo dipende da diversi fattori e generalmente non è legato a una pianificazione temporale fissa: utilizzo attivo del dispositivo, notifiche push alle app, aggiornamenti basati sulla posizione e capacità di apprendimento di iOS 7 per ottimizzare in base ai modelli di utilizzo delle app.
Da iOS: Comprensione del multitasking (grassetto corsivo mio):
Certain tasks or services can continue to run in the background. To lessen the affect on battery life, normal app background refreshing is scheduled for efficient times, such as when your device is connected to Wi-Fi, plugged into a power source, or being actively used.
You can adjust your apps' background activity in Settings > General > Background App Refresh.
When Background App Refresh is on, apps that take advantage of this feature can refresh themselves in the background. For example, an app can check if new content is available and download the updates, or retrieve the updated content in the background when it receives a push notification, so the new content is ready for viewing when you launch the app. Apps can also schedule background refreshing based on your location. iOS learns patterns based on your use of the device and tries to predict when an app should be updated in the background. It also learns when the device is typically inactive, such as during the night, to reduce update frequency when the device not used.
Per migliorare la durata della batteria, puoi disattivare selettivamente l'aggiornamento in background per determinate app da Impostazioni > Generale > Aggiornamento app in background.