Apple claims that iPhone 3GS offers up to 5 hours of talk time on 3G (12 hours on 2G), 5 hours of Internet use (9 hours on Wi-Fi), 10 hours of video playback or 30 hours of audio playback and finally 300 hours of standby time.
However, you might have observed significantly lesser battery life based on your usage pattern.
Apple has some useful tips on how to extend battery life of iPhone and iPod Touch.
So if your biggest complaint about the iPhone is that you need to keep recharging it regularly then you will find this article useful.