A type UF cable is run to the residenial lamp post light that is fed from the 120 volt, DM, GFCI protected branch circuit that is buried in earth without additional signal protection. What is the minimum allowable burial depth ?

A type UF cable is run to the residenial lamp post light that is fed from the 120 volt, DM, GFCI protected branch circuit that is buried in earth without additional signal protection. What is the minimum allowable burial depth ?