Really depends on iPhone model and how old it is. As well as your system settings and how many apps are left open in the background. You can google search for more ways to save battery, but the main ones are turning off GPS in the app, or turning of 3g/cellular/wifi/bluetooth/etc. Double tap Home button and swipe up to close BG apps which may be using battery.
We haven’t done definitely battery life tests with Pocket Earth, but we have done careful battery profiling and spent hundreds of hours optimizing it for reduced battery drain. We are confident it uses less battery than our competitors 🙂
Sorry, can’t say for a phone. But, on my iPad 2 (that good ol’ thing) I have had PE open for about seven to eight hours several times. About half that time, the iPad was asleep (lunch time, local museum visit, etc.) but not turned off. About a third of the time, we were using the GPS function in PE to navigate.
I turn off/disable the GPS in PE when, for instance, we can see we’re going to be walking along a trail for fifteen minutes without any need for the GPS.
One other app (PDF Expert) was running the whole time, for consulting PDF docs of notes, a guidebook, etc.
In all these cases, I began at 100% charge and ended with at least ten percent.
All wireless functions were turned off in settings. We were offline! Pretty much no need to be online with PE and a guidebook or two.