A permit issued under section 17 shall be valid till the expiry of such period, not exceeding one year, as may be prescribed, from the date of issue thereof, or till the recruitment or the persons for whose recruitment such permit is issued is completed, whichever is earlier:
     Provided that where the holder of the permit has been unable for sufficient cause, to complete such recruitment before the expiry of the prescribed period, the prescribed authority may, subject to rules made in this behalf, extend the period of validity of the permit by such further period or periods, not exceeding three months at a time.