The criterion for ensuring formation of a through hole is an important aspect of laser drilling. Close observation of the process has indicated that drilling of the substrate is achieved in six successive microstages. The stages have been time resolved and the behaviour of laser induced plasma (LIP) during the microstages of drilling has been carefully examined. The LIP behaviour during the percussion laser drilling has been explained with the help of the contemporary understanding of plasma. This paper proposes a drilling completion criterion that is based on the behaviour of LIP during laser percussion drilling of SUPERNI 263A with a Nd-doped yttrium aluminium garnet laser. The completion of formation of a hole is marked by the momentarily increased luminescence of the plasma plume emanating from the laser beam exit side of the workpiece during drilling. The criterion has been experimentally verified. The criterion is versatile and can help in augmenting productivity during manufacturing.