  Estimates suggest that while FRII jets appear to have lifetimes constrained
to hundreds of millions of years, radio galaxies with FRI jets appear to be
longer lived. We illustrate the nature of this time constraint from model
perspectives, showing how compatibility between theory and data match in a way
suggesting a key difference between active galaxies whose engines are
characterized by accretion onto co-rotating versus counter-rotating black
holes. We calculate a range of timescales for counter-rotating black holes for
a range of accretion rates compatible with theory which we then compare to
data. The validity of these timescales constitutes the most powerful recent
piece of evidence for considering counter-rotation between black holes and
accretion disks in high energy astrophysics.
