Evolution of the PWN radius (blue) and mass in the outer PWN shell (red) for the standard one-zone thin-shell formalism (dashed) and in the case of a thick homogeneous fully developed filamentary layer (solid). The yellow dots represent the moment when reverberation begins t_beg,rev. From top to bottom: models C0, S1, and S4. The dotted (green) curve describes the trajectory of the reverse shock for an SNR with δ = 0 and ω = 12. The characteristic mass is by definition the mass M_ch of the SNR ejecta, while the characteristic radius R_ch can be derived from the SNR energy, ejecta mass, and ISM density (Bandiera et al. 2021).