Hello, I am thinking about starting a business and I am wondering if there is a guide anywhere that explains about how much airbrush ink it takes to cover standard sized cakes (rounds, squares, rectangles)?

I don't believe I have ever seen a chart or guide.  This ultimately depends on what you are brushing, coverage, usage, etc.  Are you expecting to use the airbrush all the time, only some, .....  If you are trying to determine how much to buy to start, I would just buy standard bottle sizes and keep track of usage.  I have checked with Americolor and KromoKote both and they tell me airbrush colors do not expire.  They do not lose their strength in color at all.  I didn't know if that was one of your concerns as well.