if an iphone is fully charged but not in use, when will the battery run out?

That's called "self-discharge", and takes place slowly. Starting with a fully-charged (and fairly new) battery, it could sit, (powered off) for probably a couple of weeks before it self-discharged down to below 10%.

EDIT: If it's on, it will have to keep in touch with the nearest cellphone tower. How much power that takes depends on how far away that tower is. For an iPhone (which is inherently a high power device) I might guess 2 - 5 days. (But I haven't tested this.)

EDIT, 24-AUG-14: A lot also depends on the Apps you have installed. Some wake up periodically to do something; that will drain the battery. If you're concerned about maximum standby life, you should also turn off Bluetooth.

Depends on the iPhone & the health of the battery. Assuming its a 5s, 250 hrs/or just about 10 & 1/2 days (again assuming it is not doing anything)

A long time, as mine doesnt really even drop 1% overnight as I never shut mine off. (I just mute it at night w/ the corner switch)

hello

after a few days.....