Evolution of the PN toy model described in Sect. 7 for PNe having progenitor masses (from right to left) of 2 M_⊙ (red), 2.5 M_⊙ (green), 3 M_⊙ (blue), 4 M_⊙ (cyan), 5 M_⊙ (magenta). For 1 M_⊙, the Hβ luminosity would be too small to be detected. The black box in the left panel indicates the observed range for our PNe. The right panel indicates the time evolution of L(Hβ) since the formation of the PN model. It illustrates that the chance to observe a PN having a progenitor mass of 2 M_⊙ is much larger than for 2.5 M_⊙.