Studies of glacial isostatic adjustment (GIA) often use paleoshorelines and present-day deformation to constrain the viscosity of the mantle and the thickness of the elastic lithosphere. However, different studies focused on similar locations have resulted in different estimates of these physical properties. We argue that these different estimates infer apparent viscosities and apparent lithospheric elastic thicknesses, dependent on the timescale of deformation. We use recently derived relationships between these frequency dependent apparent quantities and the underlying thermodynamic conditions to produce predictions of mantle viscosity and lithospheric thickness across a broad spectrum of geophysical timescales for two Antarctic locations (Amundsen Sea and the Antarctic Peninsula). Our predictions are constrained by input from seismic tomography, require the self-consistent consideration of elastic, viscous, and transient rheological behavior and also include non-linear steady state viscosity, which have been determined by several laboratories. We demonstrate that these frequency dependent predictions of lithospheric thickness and apparent viscosity display a significant range and that they align to first order with estimates from GIA studies on different timescales. We suggest that observational studies could move towards a framework of determining the frequency dependence of apparent quantities – rather than single, frequency independent values of viscosity – to gain deeper insight into the rheological behavior of Earth.