A coat of clearcoat is 1.5mils to 2mils, so I am guessing each layer of auto paint would be about the same.  But since paint is thicker I would go with 2mils.  That converts to 500 coats to make a 1" thick layer.  I am relatively sure each layer will have to dry before adding another layer.  
Using auto enamel which dries fast would allow for several coats per day, maybe 2 or 3, really I have no real clue.  But for simple math, let's say 5 coats per day.  That equals 100 days to get a 1" thick bar.  This is my thoughts.  1, Get some large baker's pans, line them with wax paper.  Buy a few quarts of various colors.   Line up the trays so you can spray paint 4 or 5 or as many as you want.  Spray a layer in each, allowing them to  dry as you proceed changing colors and adding layers.  Again I am guessing but spray 2 or 3 layers of one color before switching to another.  Alternating the 2 or 3 with maybe 4 or 5 just to keep it random.  I believe that enamel auto paint can be recoated after a little over an hour.  
Do I have a clue?  Nope, but in the past I have asked a nephew a million questions about trying this.  He said the idea is crazy.   But if you try to make a small amount, you will spend hours cleaning equipment, the same hours if you spay 100 square feet or 1 square foot.  On the other hand, if it flops, you wasted a lot more money