Hi there,How old is your iPhone?
is your iPhone connected or disconnected to the 3G network?
The iPhone's battery power usually only lasts for around 24 hours when moderately used.
Other settings like the frequency with which you retrieve email and the number of email accounts you auto-check can both affect battery life.
As the battery becomes older, its ability to hold charge will decrease, and you will see a decrease in the length of time the battery holds its charge.
You can turn off Wi-Fi and Bluetooth to save battery life, as well as using airplane mode when you have no cellular cover.
Adjusting the brightness to make the screen dimmer will also help conserve the battery life.
Using 3G cellular networks loads data faster, but may also decrease battery life, especially in areas with limited 3G coverage.
You should also let the battery completely run down before recharging it to maintain it properly and help it keep its charge for longer.