This ultimately depends on how much equipment the DJ is bringing. The more equipment you need the DJ to provide (ie: separate sound system, video projector, lighting effects, etc) the more time you should allow. On average it takes about 30 minutes for simple sound system setups or an average of about one hour for the average sound and lighting setup.
If you are not having the event at a private residence a very important item to look over is the contract with your venue. Some people will book a hall, hotel or other similar location for 5 hours thinking they also have time before and after those 5 hours to setup and teardown. That is not always the case; some venues will charge extra for that time. Make sure that you schedule enough time for the DJ to properly setup and teardown. If the DJ has to rush to setup his or her equipment there is an increased likelihood that he or she will make a mistake during setup. That could require troubleshooting a problem or problems during your event as opposed to working out all the bugs during setup.
If you were told by another DJ that he or she can setup in 5 or 10 minutes you have to ask yourself, what in the world are they using? A boom box? One of the longest processes of setting up is the wiring off all the different audio and lighting components. In addition to the wiring a professional DJ will also take extra time to neatly tuck away cords from the view of guests. It may look simple enough at a glance but there is a lot more work into setting up and tearing down than the average person realizes.