By default, the LG screen turns off automatically after 30 seconds. The screen off time can be changed or deactivated. The latter causes the screen to stay on permanently. The following times can be selected on the LG device:

Android 9 | LG UX 9.0
Screen Timeout -  Open the  Settings
Screen Timeout -  Tap on  Display
Screen Timeout -  Tap on  Screen timeout
Screen Timeout -  Choose desired  Tiemout
  1. Open the Settings
  2. Tap on Display
  3. Tap on Screen timeout
  4. Choose desired Tiemout

LG Instructions