[Imp]

Base it on what you know there cant be any hard & fast rules, visibility can be anything from a few metres to what 30 miles? from my experience.

Night with no moon for example, heavily populated area as in city glow or even in the city visibility is way more than if you were in the middle of nowhere.
True darkness is amazing you would be lucky to make out a tree line at 100m you know somethings there & the brain finally makes sense of it. Seeing a man at that treeline not a hope in hell.
Get under the trees you can probably see 3-4 arm lengths if its a thick canopy.
I would assume something moving would be detectable at greater distance as you will pick up the motion. Also as a thought if it was your natural environment you might well see better especially if you know the area as your brain would make sense of what your seeing.

The night sky is amazing its full of stars.