What is the minimum depth in feet that a grounding rod must be driven into the ground?

Ground rods in the U.S. on a residential home are 8 foot long and are driven 8 feet into the ground. The top of both ground rods should be flush or just under the surface. One ground rod is placed directly under the meter base and another one is placed 6 feet to either side. They are connected together and grounded to the meter base with the correct size ground wire based on the amperage of the serviced being installed.