  We present a detailed study of the long-timescale X-ray variability of the
Narrow Line Seyfert 1 (NLS1) Galaxy MCG-6-30-15, based on 8 years of frequent
observations with the Rossi X-ray Timing Explorer. Combined with short
timescale XMM-Newton observations, we derive the powerspectral density (PSD)
covering 6 decades from ~10^{-8} to ~10^{-2} Hz. As with NGC4051, another NLS1,
the PSD of MCG-6-30-15 is a close analogue of the PSD of a Galactic Black Hole
X-ray binary system (GBH) in a `high' rather than a `low' state. Like high
state GBHs, its PSD is better fitted by a smoothly bending rather than sharply
breaking powerlaw model and the break frequency is 7.6^{+10}_{-3} x 10^{-5} Hz.
Assuming linear scaling of break frequency with mass, we estimate the black
hole (BH) mass in MCG-6-30-15 to be ~2.9^{+1.8}_{-1.6} X 10^{6} solar masses.
We also derive the BH mass using a variety of new optical observations and find
a value between 3.6 and 6 x 10^{6} solar masses, consistent with the mass
derived from the PSD. Combining these data with revised reverberation masses
for other AGN we update the BH mass/break timescale diagram. The observations
are consistent with NLS1s having shorter break timescales, for a given mass,
than broad line AGN, probably reflecting a higher accretion rate. However the
data are also consistent with all of the X-ray bright AGN being high state
objects. This result may simply be a selection effect, based on their selection
from X-ray all sky catalogues, and their consequent typically high X-ray/radio
ratios, which indicate high state systems.
