  We study the time evolution of molecular clouds across three Milky Way-like
isolated disc galaxy simulations at a temporal resolution of 1 Myr, and at a
range of spatial resolutions spanning two orders of magnitude in spatial scale
from ~10 pc up to ~1 kpc. The cloud evolution networks generated at the highest
spatial resolution contain a cumulative total of ~80,000 separate molecular
clouds in different galactic-dynamical environments. We find that clouds
undergo mergers at a rate proportional to the crossing time between their
centroids, but that their physical properties are largely insensitive to these
interactions. Below the gas disc scale-height, the cloud lifetime obeys a
scaling relation of the form $\tau_{\rm life} \propto \ell^{-0.3}$ with the
cloud size $\ell$, consistent with over-densities that collapse, form stars,
and are dispersed by stellar feedback. Above the disc scale-height, these
self-gravitating regions are no longer resolved, so the scaling relation
flattens to a constant value of ~13 Myr, consistent with the turbulent crossing
time of the gas disc, as observed in nearby disc galaxies.
