Observation times required for different period spacings, ΔP, in a period range relevant for the PSPs of merger products in our work. The horizontal dashed silver lines indicate the number of years on the y-axis. The vertical solid and dashed lines show the maximum period (i.e. the period for the mode at n_pg = −200) for the (ℓ, m) = (1,  0) and (ℓ, m) = (2,  0) modes, respectively, for the non-rotating 6.0 + 2.4 M_⊙ (black) and 9.0 + 6.3 M_⊙ (grey) merger products (see Figs. 3 and 5).