I found out that the customer's choice of item to buy is completely random and that TL has programed to "give back" some food items in order to slow down the game, which tends to make trying to figure the consumption rate less of a factor. You can not predict the consumption rate for any product given the way the program works. Once you have reached a level where production exceeds consumption it doesn't matter how long it takes to produce product. I have over 30,000 plates of each item I have ready to sell. I have more than enough items to sell while I prepare even the items with the longest prep times.
What you can do is prepare only the items that either give you the most number of coins or XP points, which ever you are trying to achieve. Produce them in large enough quantities to keep the customers buying only those items and still give you time to prepare them. Don't bother with other products if you don't need them.