Determining the thickness of the lithosphere in any given setting combines uncertainty in both the observational method and laboratory‐derived understanding of mantle rheology. The many observational and modeling criteria across geophysical subfields for plate thickness lead to significant differences in plate thickness estimates depending on the process of interest, be it seismic wave propagation or relaxation in response to changes in loads—from earthquakes, ice sheets to volcanoes—or convection. This paper proposes a framework in which to model and interpret upper mantle mechanical structure smoothly across the full spectrum of geophysical timescales. We integrate viscous, elastic, and linear anelastic constitutive models and calculate the mechanical response from convective to seismic wave timescales (i.e., 0 to infinite frequency or, in practice, 10⁻¹⁵ to 1 Hz). We apply these calculations to 1‐D thermal structures and determine the normalized complex viscosity, a quantity that shows clearly the role of transient creep in weakening rock relative to the associated Maxwell rheology. Using various criteria for the lithosphere‐asthenosphere boundary, we show that the apparent plate thickness will be thicker at higher frequencies than at lower frequencies. Additional calculations for nonlinear Maxwell behavior (dislocation mechanisms) demonstrate significant changes in the apparent plate structure, decreasing the long‐term plate thickness, consistent with observations. Other effects such as dislocation damping (associated with a steady‐state dislocation structure), melt, water, major element composition, and grain size are not included here but, when incorporated into this framework, will significantly change the full‐spectrum plate thickness predictions.