My fan powers "hot" through the oil temp switch (like described by Robert M above), even after ignition switch is turned off. At one time I considered rewiring the fan though an ignition circuit but have since decided I'm better with letting it run after the engine is turned off (and not because I think it sounds cool).
Over the past 15+ years/35000 miles I have replaced my horizontally mounted fan twice. And based on the durability of the first two fans, I think my third fan has about reached the term of its life expectancy. These have all been the standard fans found in the familiar VW parts catalogs.
As a matter of conscious habit, when I turn off the engine I want to hear the fan wailing in the back. When it starts sounding wimpy, that's my clue it's time for some routine replacement maintenance.
It doesn't bother me that the fan stays on after the ignition is turned off (which is also typical of many modern cars). I much prefer knowing that I don't have a dead fan before blasting off on a long drive on a hot day.