the Apple store can check your battery stats via a test they have. They link to your phone in the store. I did this with my iPhone 4.
Just a few BIG variables include:
How much music do you listen to? Music has very low drain, so the usage is high and battery change in minimal.
How strong is your signal? Weak signal strength means higher battery drain.
And, of course, the ones mentioned like GPS/location, screen brightness, etc.
I never have had the high numbers you see on the forums unless I use it as in iPod or something. I spend a lot of time on safari, and typically get 5-7 hours of use.
On mine, there is probably an hour or so music listening, there is a lot of email and text and phone calls. A small amount of Safari (but that's what my iPad is for). I manually control brightness and keep it comfortable but not overly bright.
GPS/location is restricted to core apps, most third party apps don't need it to work.