Mine regularly stays on 100% for 8 hours when I barely use it. It said I had 22 days left after dropping to only 94% after 28 hours of basically no use. It was only down to 40% after 3 whole days, which basically only consisted of 2.5 hours worth of phone calls and nothing else.
I have had nothing but bad battery perf with my phone...this leads me to believe its some sort of app or software issue in the OS that sometimes goes rampant.