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

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

After a few days…

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.)

A long time, as mine doesn't really even drop 1% overnight as I never shut mine off. (I just mute it at night w/ the corner switch)

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)