When does an automatic app update happen on iOS?
When the device checks for updates, it builds a list of all apps that need updating. All of those updates will be installed assuming these criteria are met:


The iOS device must be on and/or asleep.
The iOS device must be connected to power and/or battery is almost full. 
The iOS device must be connected to a cellular data network and/or Wi-Fi.
Updates larger than 100MB will only be downloaded over Wi-Fi, never cellular. You can also disallow updating over cellular. 
If device is in Low Power Mode it will not update