According to Official Guinness Records,
The fastest time for producing 13 loaves (a bakers dozen) from growing wheat is 8min 13.6sec, by representatives from Wheat Montana Farms Bakery at Three Forks, Montana, USA on 19 Sep 1995. They used 13 microwaves to bake the loaves.
For a complete list of 1995 records, please visit 1995 Guinness Records in United States.