  The mechanism by which galaxies stop forming stars and get rid of their
interstellar medium (ISM) remains elusive. Here, we study a sample of more than
two thousand elliptical galaxies in which dust emission has been detected. This
is the largest sample of such galaxies ever analysed. We infer the timescale
for removal of dust in these galaxies and investigate its dependency on
physical and environmental properties. We obtain a dust removal timescale in
elliptical galaxies of $\tau$ = 2.26 $\pm$ 0.18 Gyr, corresponding to a
half-life time of 1.57 $\pm$ 0.12 Gyr. This timescale does not depend on
environment, stellar mass or redshift. We observe a departure of dusty
elliptical galaxies from the star formation rate vs. dust mass relation. This
is caused by the star-formation rates declining faster than the dust masses and
indicates that there exists an internal mechanism, which affects star
formation, but leaves the ISM intact. Morphological quenching together with
ionisation or outflows caused by older stellar populations (supernova type Ia
or planetary nebulae) are consistent with these observations.
