My games no longer have any clerics in them. But back when I had them, they turned undead. How exactly, asked Brett Eliot on Google+.
I ruled that LOS is important. I imagine the holy symbol shining some sort of holy light which hurts the undead. It’s that light which disintegrates them, too. Thus, distance is not a problem, in theory. But the undead can always take cover and so it depends. In a flat desert the army of undead will appear over the horizon and immediately freeze when they see the lone cleric, miles away, turning them. And slowly, in the milky twilight his holy light one zombie after another starts to melt, disintegrate. Do they rush forward, hoping to reach the cleric before all being destroyed, or do they build man shields to protect them, or do they take cover and wait for another day? Clearly, that explains why the undead prefer to exist underground. Less exposure to random holy photons.