I don't think you can or I couldn't find it. I wanted it so that I could show my clock during the night. I ended up getting a 3rd party program that will stay on.
Daydream under display settings. It's like a screen saver while charging. When you plug your phone in let the screen shut off by itself. Then all you have to do is touch the screen to wake it up. If you want to have the clock show you need to dl the new Google clock.