The brightness of the screen and the amount of time it takes to automatically power down after no use can be set in Display Settings.

Choose Menu>Settings>System>Display Settings

Brightness provides a slide bar for manually setting the Brightness or an option to have the system automatically set the brightness.  If you prefer the device to adjust the brightness automatically, check the option.

Screen Timeout offers settings from 15 seconds to 1 hour.  This setting determines how long the screen will stay on after the last activation.