Quick answer
Deep enough that the next person to dig finds soil, not your cable. IS 1255, the Indian code for laying power cables up to 33 kV, measures depth from ground level to the top of the cable and raises it with voltage: roughly 0.75 m for LV up to 1.1 kV, about 0.9 m for 3.3 to 11 kV, and about 1.05 m for 22 to 33 kV. Under roads and crossings, go deeper and pull the cable through a pipe or duct.
Depth alone is not enough. Bed the cable in about 75 mm of clean sand, cover it with another 75 mm, lay bricks or protective tiles over that, and run marker tape at the surface. Use an armoured cable for direct burial and earth the armour.