The instructor pilot said that he had flown the helicopter on the flight prior to the accident session for about 1.2 hours. The helicopter began that flight with full fuel tanks and it was not refueled after landing. The instructor stated that he performed the preflight inspection with the student on the accident flight and visually looked inside the fuel tank and determined that about one hour and forty minutes of fuel was on board. He said that he did not use a measuring device to quantify the amount of fuel in the tank, but rather visually estimated that the amount he observed matched the fuel gage. After departure from Burbank, the flight proceeded to the local practice area where the one hour dual student was given a lesson on primary flight maneuvers. The flight was on approach to the Burbank airport after one hour and twelve minutes when the low fuel level warning light illuminated. The instructor noted that the engine quit immediately thereafter and he executed an auto rotation to a city park. The helicopter landed hard, bending the tail boom. No fuel was found in the tank after the accident.