Power to center console USB
 

I've searched the manual and I can't find a description of the power supply provided to the USB plug when the X5 engine is NOT on. Can anyone tell me how long power continues to be supplied to the USB outlet once you shut the engine off and remove the key?

I've got an old iPhone plugged in (basically using it like an iPod Touch) strictly for music, and I don't want to leave it plugged in at all times if it's going to drain the battery with the car off.

Sorry I should clarify...I'm driving a 2011 35i Premium, post LCI.

I do the exact same thing - keep an old iPhone 2G there all the time. No issues with battery during the 2 years.
The power is supplied for about 15 minutes, same timeout for when music/radio shuts off if you are listening to with engine off.

Even with key out of ignition, power is supplied for same amount of time radio is able to play. Slightly less than 15 minutes.

Once, while camping and strong winds were rocking our tent, my wife and 2 kids moved in to sleep in the X5 (yes, we fit in there!). Obviously, we were not locked in the vehicle. A couple times a night vehicle would wake up and the iPhone would make the ding sound and light up. Then goes back to power disconnect in a minute. So, the vehicle does wake up when not locked.
If locked, the vehicle should go into sleep mode and technically should not wake up.

P.S. your avatar picture is upside down :)

Thanks Seattle! Yes I know the picture is upside down. I loaded it from my iPhone and it showed up like that. When I'm home tonight I'll try again from my PC.