On clear days, the Chicago skyline can be seen from the Indiana shore of Lake Michigan. Depending upon where you are, that's a distance of 27 miles to 50 miles.
The very bottom of the buildings are lost below the horizon due to the curve of the earth. The actual horizon over water is around 4 miles away if you're standing on the shore - meaning you can only see the surface of the water 4 miles away. Taller objects can obviously be seen at greater distances, and standing atop a dune certainly doesn't hurt either.