The advance of the hydrate dissociation front is crucial to both the energy recovery process and the geological hazards prevention. In this work, the role of the scale of gas hydrate reservoirs in the controlling mechanisms is investigated numerically with the lab-scale and field-scale models. It is suggested that the scale of hydrate reservoir is not reliable in discriminating the three controlling mechanisms, i.e., fluid-flow (FF), heat-transfer (HT), and hydrate-dissociation-kinetics (HDK). For instance, the controlling mechanism of both the lab- and the field-scale hydrate reservoirs both might be FF as revealed in this work. Therefore, an approach of characteristic time is proposed to discriminate controlling mechanisms by employing dimensionless analyses. Also, mathematical models of the velocity of hydrate dissociation front conditioned to the single controlling mechanisms have been established, which are used to optimize the characteristic times. According to the optimized results, single characteristic times are sufficient for the HT and HDK controlling mechanisms. For instance, the time consumed to transfer the heat in the hydrate reservoirs by heat conduction through the gas phase may serve as the characteristic time of the HT controlling mechanism. As for the FF controlling mechanism, two different characteristic times should be used, i.e., a gas-phase based characteristic time for the initial stage and a water-phase based one for the later stage. The optimized characteristic times are capable of capturing the variation of hydrate reservoir controlling mechanism with geological and production parameters.