With ever increasing penetration of inverter-interfaced generators and consequent reduction in available inertial reserve, traditional allowable delay in the provision of the primary frequency reserve will threaten the security of the future grids. However, droop control with fast-enough inertia-less generators can limit the post-contingency frequency excursion. To conform with the fast-enough reserve provision requirement in a reduced and varying-inertia system, an economical clearing of the reserve requires an allowable delay heuristic corresponding to the desired system frequency excursion characteristic; while alleviating the causality dilemma introduced by the unavailability of maximum delay heuristic itself before clearing. The proposed heuristic establishes a relationship among the system parameters, such as the aggregated system inertia, regulation factor, system frequency, desired frequency response, and allowable delay margin. The Padé approximation technique is utilized for deriving the transfer function of such a system, allowing calculation of the allowable delay with various damping ratios corresponding to the dominant poles. The impact of different Padé order on the determined heuristic is analyzed. The regulation factor of the generators is calculated based on the reserve bid volume to ensure proportional injection during contingencies. The impacts of regulation factor and associated delay on the frequency response are also studied.