In iOS 7, some apps can now update their data even when the app is not open. This feature is called "Background App Refresh". How often does each app update their content using this method? Is it up to the app to determine the frequency, or does iOS 7 limit them?
But for turn-by-turn navigation, the app is in the foreground, not the background. I'd imagine the havoc that would be caused if you had 50 apps, all updating every 30 seconds. It will kill the battery very quickly. Having a single app in the foreground update every second is not nearly as bad.