iDimLight let’s you modify the back light dimming time. When you start the application, a popup opens and asks you to enter the number of seconds before you like the back light to be dimmed. You can enter any number of seconds you like. If you’d like to disable the app again, just start the app and enter -1. After you have entered the seconds, the app will ask you to restart SB.
» iDimLight - Adjust your iPhone light dimming