  In this work, we introduce an information-theoretic approach for considering
changes in dynamics of finitely dimensional open quantum systems governed by
master equations. This experimentally motivated approach arises from
considering how the averages of system observables change with time and
quantifies how non-stationary the system is during a given time regime. By
drawing an analogy with spiral motion, we are able to further investigate
regimes when such changes are negligible according to the logarithmic scale of
time, and thus the system is approximately stationary. While this is always the
case within the initial and final regimes of the dynamics, with the system
respectively approximated by its initial and asymptotic states, we show that a
distinct regime of approximate stationarity may arise. In turn, we establish a
quantitative description of the phenomenon of metastability in open quantum
systems. The initial relaxation occurring before the corresponding metastable
regime and of the long-time dynamics taking place afterwards are also
characterised. Furthermore, we explain how metastability relates to the
separation in the real part of the master equation spectrum and connect our
approach to the spectral theory of metastability, clarifying when the latter
follows. All of our general results directly translate to Markovian dynamics of
classical stochastic systems.
