It can be done either through the keypad of via Sentinel. In Sentinel, edit the applicable profile, go to Display Options tab and look for backlight. Choose a value of anything other than indefinite for how long the backlight timer remains on after an incoming transmission is detected.
On the radio go to Menu (press lower side button) (> = Enter, or Press Down on Volume Knob) Scroll to Display Options>Set Backlight>Set Mode>Squelch>Open Squelch>Press Avoid to return to scan. This will turn the light on whenever there is audio and turn it off when audio ceases.
I personally use either 5 or 10 seconds which conserves battery life.