This is an interesting app that could come in handy. Battery Go! is an app that's all about your iPhone, iPod touch, or iPad's battery. It'll give you all kinds of information about how long your battery will last, given either the charge it currently has or the charge percentage that you set it at. If you want to see just how long your phone will last on standby, how much talk time you have left, or how much longer your video will play before the phone dies, Battery Go! will tell you.
Of course, these are all just guidelines based on the battery specs. Actual battery usage has lots of other factors involved, but as a rough estimate, Battery Go! can help. If you spend a lot of time away from a way to charge your iDevice and need some help budgeting out your power, Battery Go! can do it.
Plus, the app is currently free to download, so even if you don't plan on using it, there's no charge to have it just in case.