Dust-to-stellar mass ratio as function of luminosity-weighted stellar age of early-type galaxies detected by Herschel (black circles; Rowlands et al. 2012). The exponential fit (solid line) has the lifetime τ = (2.5 ± 0.4) Gyr and the corresponding half-life time of ∼(1.75 ± 0.25) Gyr. The typical errors (large cross, ∼0.3 dex for M_dust/M_* and ∼0.2 dex for the age) are much smaller than the range of the values. The targets of JCMT/SCUBA-2 observations are marked as green circles. Open circles denote galaxies which are within the main sequence (see Fig. 3).