How to Get Your Screen to Stay on Longer for an iPhone
The Apple iPhone contains several features that can extend your phone's battery life. Some of these features may interfere with the tasks that you need to perform with the device. If you find that the screen dims too quickly, you can adjust the brightness settings in the "Settings" menu of your iPhone. You can also adjust the overall brightness level for your phone's screen.
Tap "Settings" on your iPhone.
Tap "Brightness" to select it.
Drag the slider to adjust the brightness of your screen. To prevent your screen from dimming prematurely, select "Off" for Auto-Brightness.