Apologies if this has been asked previously, I've looked and found similar questions but not this aspect.
I've downloaded a screen saver app, Aerial Dream, which is awesome by the way, but this problem exists for both this app and ambient mode.
I can change how long the screen saver /ambient mode take to start, but I can't seem to change how long it stays on for, after about 10 minutes my screen goes to the 'no Hdmi input' screen. (using dongle)
I've tried changing the 'energy saver' setting but this doesn't seem to work, is this supposed to be the setting or is there another one I'm missing?
There should be a "sleep timer" in your Android settings. It can be named differently on different devices.
Here is some documentation on how to configure this
https://splash-tiles.com/help/screensaver.php
If your device doesn't have this in the menus, you can often use the ADB command listed on that page to configure the sleep timeout.