  We present a renormalization group (RG) method which allows for an analytical
study of the transient dynamics of open quantum systems on all time scales.
Whereas oscillation frequencies and decay rates of exponential time evolution
follow from the fixed point positions, the long-time behavior of
pre-exponential functions is related to the scaling behavior around the fixed
points. We show that certain terms of the RG flow are only cut off by inverse
time, which leads to a difference between infrared and ultraviolet scaling. An
evaluation for the ohmic spin boson model at weak damping reveals significant
deviations from previous predictions in the long-time regime. We propose that
weak coupling problems for stationary quantities can in principle turn into
strong coupling ones for the determination of the long-time behavior.
