Wow that's amazing. I've never had a battery last 5 days without needing a charge! With moderate use (email, Newsclip, BBWeather) I usually have about 70% left at the end of the day. Not bad for an OEM battery of its size. I had similar results with the extended battery in my 8700, but that battery was HUGE.
So far I haven't noticed any falloff in battery life. But then again, I too charge it up every nite, so I don't have any test data to give you. If you think it's a problem I'd contact your carrier and get a replacement battery under warranty.