Here is a handy app for the iPhone, Battery Status will show you at a glance how much time in Hours and Minutes you have left to Talk, Surf or use Multimedia services like Music,Video & Games.

I have already found it useful on long trips to charge up my phone before I set off on the next stage of my journey with enough juice to reach the other end. You can download it to your iPhone using this iTunes link to the Battery Status download page.