How long would it take a car engine, say 500 pounds of iron, to cool from 1,200 degrees F (exauhst manifold temp) to air ambient temp. of 75 degrees F.

It will approach the air ambient temperature asymptotically. The time it takes for the temperature to get within a few degrees of ambient will depend upon the heat transfer rate as well as the mass of the engine. Is it being cooled by a radiator, forced convection or natural convection?