Were Bluetooth, GPS, Wifi active ? Even if the phone is not in active use, those features help drain the battery faster. I am glad that Apple did eventually replace the phone, but that is a common user mistake.
I have had a very frustrating experience with my iPhone 5 the battery was not lasting more than 6 hours on standby. I went to the Apple store and went through the restore as new phone process. It didn't fix the problem. I returned the following week and was told that the diagnostics show that it is operating normally, that it was a computer and needed to be plugged in most of the time. I am not alone there are hundreds of pages on the Apple support communities and hundreds of thousands of views. I was finally successful in getting a new phone but I had to insist that the phone was not operating normally as we have two others in our household that would last for days on standby. I am very dissattisfied with my experience with the Apple Geniuses at the Albuquerque, NM store.