What is the normal distance the ground wire needs to be placed in the ground?

I am not an expert on this issue, but NEC code 250.53 states the following: 

(H) Rod and Pipe Electrodes. The electrode shall be installed such that at least 2.44 m (8 ft) of length is in contact with the soil. It shall be driven to a depth of not less than 2.44 m (8 ft) except that, where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or shall be buried in a trench that is at least 750 mm (2 1/ 2 ft) deep. The upper end of the electrode shall be flush with or below ground level unless the aboveground end and the grounding electrode conductor attachment are protected against physical damage as specified in 250.10.

You may want to check the following websites:

http://www.cosjwt.com/index.php?a=16

http://www.eham.net/articles/12488

http://www.dxing.com/tnotes/tnote02.pdf

ok thank you, this was helpful