Finding the Sleep Timer settings and customization is pretty easy:
Press and hold the Home Button on the Voice Remote for 3 seconds to launch the Quick Access Menu.
Scroll down and select Sleep Timer.
The Sleep Timer function turns off the TV in: 5 , 10 , 15 , 30 , 60, 90, 120, 150, 180 and 240 minutes.
Also you can go to Home > Settings > Sleep Timer.