Minimum central radius of a planetesimal belt, such that a total mass of either M_disc = 10^-5, 10^-3, 1, or 10 M_⊕, can survive, as a function of age. This is calculated using Eq. (6), assuming a steady-state collisional equilibrium, with bodies up to a maximum size D_max = 60 km and fiducial values for the other parameters taken from fits to observations of main sequence debris discs, quoted in Sect. 2.3. The results scale directly with and the time axis can be scaled by a factor involving the stellar mass, .