To be honest, I returned my phone because it would only last me 5-6 hours. I did use it heavily however, with a few hundred text messages throughout the day, emailing, browsing, and angry birds when I'm bored. But I also used Juice Defender, turned off 4 g, had screen brightness down below 25% (couldn't even see the screen outside), and pretty much every other way to conserve the battery. I bought the extended battery but it was wayy too bulky and I returned everything (even managed to get the restocking fee waived)
I bought my friends Droid X for a ridiculously cheap price and I love it. Nothing compared to the speed of the thunderbolt but it'll last me until the Bionic comes out. Oh and the battery life is sooo much better without any tweaking at ALL!! I just loaded my phone and do the same thing I did with the thunderbolt without lowering brightness, no juice defender, or any of those other needed to do things. And now i'm not afraid to use my Droid X while always constantly looking at the battery hoping it'll last me a few more hours until I get to a place where I can charge it haha.