According to CRIDA (Central Research Institute for Dryland Agriculture) classification of Tillage, Deep tillage depth is

Ploughing is the primary tillage operation used to loosen or turn the soil before seeding or planting. the main motive of ploughing is to turn over the uppermost layer of soil to bring fresh nutrients to the surface.
CRIDA (Central Research Institute for Dryland Agriculture) has classified tillage in accordance with the depth; 5 - 6 cm is regarded as shallow, 15 - 20 cm is medium and 25 - 30 cm is regarded as deep tillage.