Only got this yesterday so haven't played about enough to notice on this monitor, but on my older B6 (living room TV) the ABL kicks in after a few minutes of a static image on screen, e.g. pause movie/game, and gets brighter once you have significant differences in what's being displayed, e.g. when you un-pause what you are watching.
Just for clarity, the autodimming (ASBL) at 3 minutes can be turned off, I've done it and it works great.
See instructions here: https://hardforum.com/threads/lg-48cx.1991077/page-79#post-1044661831
Awesome thanks for the info and link to switch it off, sounds like its a good idea to leave it on though would be pretty crazy to have it happen during normal viewing of film or tv but least the options there to switch it off if it does.