Only when you use them.
You can either not install them at all (problem solved).
or only allow them to "know your location" when you explicitly launch them and only for the duration of you actively using them on screen (problem lessened).
> in iOS 11, users will be able to choose the “when in use” option at all times [when asked for permission to use their location]
Closed or in the background?
I was under the impression (in iOS 10 and earlier), that "Always" meant anytime the app was open, and "While in use" is only when the app is in the foreground. So if an app has "Always" on for its Location Services, it will stop tracking when I swipe up and quit it.
If I've been incorrect this whole time, please pass the tinfoil.
You could turn off the gps but that's too coarse: you might want it for some apps.