Airborne wind energy (AWE) systems harness wind power using devices flying in controlled patterns, with two main concepts: onboard power generation in the 'drag mode' and ground-based generation in the 'pumping cycle'. Quasi-steady state models (QSM) efficiently predict parameters like tether force and kite velocity for smaller AWE systems but the neglect of inertial forces leads to inaccuracies for bigger systems. Various formulations of quasi-steadiness exist in AWE literature, but their validity limits remain poorly understood. This research presents a theoretical framework specifically tailored to crosswind tethered flight dynamics, which is used to obtain a consistent definition of quasi-steadiness and to quantify the validity of this assumption, through extensive solution space analysis. The study finds that QSM provides reasonable accuracy for time-averaged quantities but fails to predict other aspects such as amplitude or phase. Limitations arise from the use of a steady aerodynamic model and assumptions of a rigid tether.