Q: When you turn on the defroster/defogger, is the windshield supposed to defog instantly?

This can actually take a few minutes - and depends on the temperature and level of moisture in the air inside as well as outside the vehicle. Two things happen when you use "Defrost/Defog". The air needs to be heated, and the air needs to be "dry" (low moisture content). Engine heat provides the heated air (through the heater ducts), and the AC system takes moisture out of the air inside the car while cooling it. But then this cool air requires more heat to help defog. So, as you can see - the engine has to heat up, and the AC has to work a few minutes to dry out the air inside the car. Great question!