I used the general rule that, if you could see the dash light at all when on, it was dark enough to use headlights. I never understood why people waste time with parking lights only. Are they trying to save electricity, or what? Why would you not want your car as visible as possible.
Three times now I have passed a DA in a CRV on my way to work that has non-functional tailights. The paint is awful and the thing looks like a junkyard reject. The headlights work, but are so clouded and dirty that I doubt they actually light the path for the driver. I keep wanting to call it in, but who would I call? Sooner or later some half-drunk driver will plow into the back of it (it drives below the speed limit) and save me the trouble.