Grid and system impedances are crucial inputs for the integration of renewables, evaluations of system stability and novel monitoring methods. They can be determined through various setups and excitation methods. However, the increasing use of grid-connected converters creates subperiodic effects, challenging the requirement of time-invariance in most an-alytical methodologies. This paper presents a novel indicator, the Subperiodic Impedance Variance (SIV), to quantify the impact of the time-variance of a given system. Laboratory measurements and simulations with different impedance meas-urement methods of systems with varying degrees of time-variance have been conducted to estimate provisional threshold values for the application of broadband excitation impedance measurements. Extensive field measurements were analyzed to determine real-world SIV values across various grid nodes.