First, to the OP -- the battery life on the S3, even with the stock battery and running stock OS is definitely a noticeable improvement over the Thunderbolt's battery. As a previous TBolt owner myself, I can attest to this first-hand. Obviously each person uses their phone differently, but with a similar usage profile, I'd estimate I get 1.5 to 2 time the battery life out of mine. The only problem, the S3 is so awesome, I find I use it more (which obviously is a battery killer in and of itself!).
There are a lot factors to consider. I have easily got 18+ hours on a charge before...but that's assuming I'm not using it non-stop, spending most of my time on WIFI and definitely not streaming videos. Two weeks ago, for example, I was home all weekend and generally just hanging around the house, so I pulled my phone off the charger Saturday morning and left it off until Sunday morning with around 20-30% remaining (sometimes I'll leave it off the charger just to see how far I can push it!). So that's a full 24 hours and charge to spare.
Now, on the flip side, during the week when I'm working in my office (with awful cell reception and no wifi) and using my phone much more frequently, I generally have to plug in the phone after lunch just to make it through the day.