  The description of an open quantum system's decay almost always requires
several approximations as to remain tractable. Here, we first revisit the
meaning, domain and seeming contradictions of a few of the most widely used of
such approximations: semi-group Markovianity, linear response theory,
Wigner--Weisskopf and rotating-wave approximation. Secondly, we derive an
effective time-dependent decay theory and corresponding generalized quantum
regression relations for an open quantum system linearly coupled to an
environment. This theory covers all timescales, and subsumes the Markovian and
linear-response results as limiting cases. Finally, we apply our theory to the
phenomenon of quantum friction.
