Our local PoCo went behind their own office and drove 10 8' ground rods and never got 25 ohms. So they let us get by with two now. I worked on a 1200 amp 480v with 5 rods 10' apart cadwelded to 4/0 copper in addition to a clamp on a 4" galvanize water line thirty feet in direct contact with the earth and never got 25 ohms. But the key here was undisturbed earth, seems any fill really messes thing up.
