How long does an iOS device have to be left at the Setup Assistant screen before automatically turning itself off?

When either turning a new iOS device on for the first time or choosing to erase all content and settings from a previously-used device, upon booting for the first time after either of the above has been performed the iOS Setup Assistant is displayed. When erasing a test device, I've noticed that the device appears to turn itself off after being left on this screen for a certain amount of time. Would anybody happen to know what this amount of time is? Is it a case of being left inactive for a certain amount of time, or is this instead triggered when the battery level falls below a certain percentage?