It depends so much on how you use it and what push and/or fetch settings you have on email and stuff. Also what sort of reception area you're in for the mobile signal, whether you are on 2G or 3G, and what the brightness is set to.
As an example of how a frugal setup can maximise battery life, here are my stats right now. I'm at 59% and I already have 4 hours 36 minutes of usage and 2 days 17 hours of standby. Brightness set to about 65% (under the "e" in "Brightness") and no music usage or phone calls, it's mostly WiFi browsing with a tiny bit (a couple of minutes) of 2G mobile data access. WiFi is permanently enabled although it doesn't really need to be so I could probably eke out the battery life even more if I could be bothered.
I keep my phone locked to the 2G network, never access email from my phone, have calendar and contacts set for manual update and rarely use the phone or data functionality at all. I'm probably a very untypical user but for me my iPhone is essentially a PDA device that I use to read ebooks, listen to music, take notes, store maps, spreadsheets and other key information on, and play games. My device just happens to also have a phone feature in it which is very useful in emergencies or when running late for a meeting but is rarely used apart from that, and the occassional mobile access to Wikipedia to settle arguments in pubs.
- Julian