  The processes of star formation and feedback, regulating the cycle of matter
between gas and stars on the scales of giant molecular clouds (GMCs;
$\sim$100pc), play a major role in governing galaxy evolution. Measuring the
time-scales of GMC evolution is important to identify and characterise the
specific physical mechanisms that drive this transition. By applying a robust
statistical method to high-resolution CO and narrow-band H$\alpha$ imaging from
the PHANGS survey, we systematically measure the evolutionary timeline from
molecular clouds to exposed young stellar regions on GMC scales, across the
discs of an unprecedented sample of 54 star-forming main-sequence galaxies
(excluding their unresolved centres). We find that clouds live for about
$1{-}3$ GMC turbulence crossing times ($5{-}30$Myr) and are efficiently
dispersed by stellar feedback within $1{-}5$Myr once the star-forming region
becomes partially exposed, resulting in integrated star formation efficiencies
of $1{-}8$%. These ranges reflect physical galaxy-to-galaxy variation. In order
to evaluate whether galactic environment influences GMC evolution, we correlate
our measurements with average properties of the GMCs and their local galactic
environment. We find several strong correlations that can be physically
understood, revealing a quantitative link between galactic-scale environmental
properties and the small-scale GMC evolution. Notably, the measured CO-visible
cloud lifetimes become shorter with decreasing galaxy mass, mostly due to the
increasing presence of CO-dark molecular gas in such environment. Our results
represent a first step towards a comprehensive picture of cloud assembly and
dispersal, which requires further extension and refinement with tracers of the
atomic gas, dust, and deeply-embedded stars.
