How much wear is placed on an engine when it idles for long periods of time?

I am a Chief of Security for a hospital and our patrol vehicle is a 2009 Jeep Grand Cherokee, 6 cylinder. The engine has been repaired several times and it was just reworked about a month ago, now the number cylinder has a miss. One of my shift supervisors advised that the vehicle is sitting idle for hours on end, without being moved. How damaging is that to the engine? Does the oil need to be changed before the basic 3000 mile use? What information can you give me that will help me make the best decisions to improve the life of our patrol vehicle?