It ranges between 10 to 30 minutes. It is attached to a TV via HDMI. The monitor will go black. Moving the mouse of tapping the keyboard of the Raspberry Pi will "wake up" the screen pretty quickly.
It is running Raspbian OS as a standard image. The resolution is set to CEA mode 32 1920x1080p.
How do I stop it going to sleep? It is for a monitoring dashboard which would ideally run without interference :-)