An experimental study of burning of natural gas puffs generated by a fully modulated jet, has been conducted. The present study concentrated on the effects of duty-cycle and puff volume, or equivalently the injection time, on the flame length. It is observed that when individual puffs are well-separated, considerably shorter flame lengths, by as much as a factor of four, are obtained compared to a steady jet flame. Furthermore, the flame length of individual puffs scales with the initial volume of puff, in agreement with the previous studies of buoyant puffs in aqueous media. For a given puff volume, as the duty-cycle is increased beyond a given value, the flame length tends to rise rapidly due to the interaction among neighboring puffs. A dimensionless parameter is found which indicates the transition of puff characteristics.