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.