Gasand pebble masses as a function of time (top panel) and the mass accretion rates of gas and pebbles (bottom panel). The different pebble models are explained in the main text. Protoplanetary discs with large pebbles – of mm or cm sizes or with constant Stokes number of 0.1 – drain their pebbles out of the disc in much less than 1 Myr. The mass-accretion rates of such large pebbles is very high during this period. The models with a lower Stokes number are better able to maintain their pebbles for the entire life-time of the protoplanetary disc.