A reply from the Phone Geek (he works for ATT Wireless)
Since the iPhone uses a lithium battery, you need not wait to charge it. I would just charge it each night; one it is charged, yhou are not discharging the backup battery. I would also recommend turning off the phone when not in use. Cell phones are constantly "looking" for service and especially if there is not service, they will use even MORE power. Also, the display is one of the largest users of power -- same for any GPS device: the display is the real power user.
