According to the scaling predicted by FGR, 2T 1 scales as N/(N -1) when √ N g c is fixed, scales as N 3/2 /(N -1) when g c is fixed, and scales as g c when N is fixed. As one can see, the least square fittings match the HEOM data for all three panels in Fig. and show that our scaling arguments are correct. Furthermore, we see that the FGR expression overestimates the T 2 value by only 40 fs, likely due to ignoring the other contribution of decoherence (that further reduces T 2 .) Thus, we note that Eqs. ( ) and ( ) not only reproduce the scaling of T 2 with respect to the system parameters, but they also provide a reasonable estimate for the actual coherence lifetimes predicted by exact quantum dynamics. We expect these equations to be of use in interpreting experimental results that couple many molecules strongly to a cavity, such as polariton spectral linewidth. Further, we demonstrate the robustness of the prolonged coherence ρ +-(t) when explicitly considering cavity loss. To incorporate the cavity loss effect, we couple the cavity mode with a lossy environmental DOF corresponding to the photonic modes outside the cavity (far field modes). This part of the Hamiltonian is expressed as