The real answer is "as deep as physically practicable". In some cases that
kind of depth is impossible depending upon where the layers of shale and
rock are. So, the deeper the better.
If you can't make it deeper, then try to make the rod have as much surface
area in the ground as possible. I've had rods bent in two places so there
was a horizontal part making up some of the difference in depth. As long
as it's underground, it won't matter.
As a tip, it also helps to have a hose watering the ground as you hammer
it in. Makes it a bit easier going.