Predicting the timing of droplet coalescence remains a fundamental challenge because the transition from thin film drainage to interface rupture is not specified by existing models. While the Stokes-Reynolds-Young-Laplace (SRYL) framework accurately describes hydrodynamic film thinning, it lacks a criterion for the onset of rupture in a chemically complex environment under external compression. To bridge this gap, we employ a contactless magnetic Kelvin force-based apparatus to drive precisely controlled, head-on droplet collisions to investigate the drop-drop coalescence behavior. Using this platform, we systematically measure the coalescence residence time under varied material properties. To facilitate the analysis of these results, we present a dimensionless similarity formulation that lumps the dependence of drainage dynamics on viscosity, interfacial tension, droplet size and the compressive Kelvin force. This formulation collapses the drainage process onto a single dimensionless master curve, where the drainage curve is uniquely determined by a dimensionless time. By projecting the experimentally measured coalescence residence time onto this master drainage curve, we derive the corresponding rupture thickness for various conditions. The resulting scaling relations provide a quantitative description of droplet coalescence and identify a characteristic hydrodynamic length scale governing thin film rupture. These findings establish a robust framework for modeling and evaluating droplet coalescence in complex, surfactant-laden multiphase systems with highly concentrated electrolytes.