On average you can expect between 2 and 3 hours of battery life without an additional batter. However, the battery burn time of your iPhone depends on many other factors out of our control, including the number of other apps running in the background of the phone, what kind of other iOS functions are active while the app is running (eg. Bluetooth, 3G data, WiFi, PING, etc.), ambient temperature, battery age and charge cycles.
One thing quite sure though: if everything is equal, our app compares quite favorably with similar cycling apps in terms of power consumption, despite the fact that the LCD screen is always on.
If you find you need longer battery life, you could consider using our ReeCharge Case for iPhone http://store.thinkbiologic.com/reecharge-case-for-iphone.html .