Yup, no setting. Totally automatic. I'm not sure if it's tied to the Auto-Brightness setting for the overall display, but yeah, in low-light situations the AOD will be very dim and, if selected, it will turn off in pitch-black environments if it thinks it's inside a pocket.
The S9 does have an Auto-Brightness setting which allows you to either select this mode, or manually adjust the brightness by double-tapping the clock on the AOD to adjust.