  We apply the viscous decretion disc (VDD) model to interpret the infrared
disc continuum emission of 80 Be stars observed in different epochs. In this
way, we determined 169 specific disc structures, namely their density scale,
$\rho_0$, and exponent, $n$. We found that the $n$ values range mainly between
$1.5$ and $3.5$, and $\rho_0$ varies between $10^{-12}$ and
$10^{-10}\,\mathrm{g\,cm^{-3}}$, with a peak close to the lower value. Our
large sample also allowed us to firmly establish that the discs around
early-type stars are denser than in late-type stars. Additionally, we estimated
the disc mass decretion rates and found that they range between $10^{-12}$ and
$10^{-9}\,\mathrm{M_{\odot}\,yr^{-1}}$. These values are compatible with recent
stellar evolution models of fast-rotating stars. One of the main findings of
this work is a correlation between the $\rho_0$ and $n$ values. In order to
find out whether these relations can be traced back to the evolution of discs
or have some other origin, we used the VDD model to calculate temporal
sequences under different assumptions for the time profile of the disc mass
injection. The results support the hypothesis that the observed distribution of
disc properties is due to a common evolutionary path. In particular, our
results suggest that the timescale for disc growth, during which the disc is
being actively fed by mass injection episodes, is shorter than the timescale
for disc dissipation, when the disc is no longer fed by the star and dissipates
as a result of the viscous diffusion of the disc material.
