  We report the analysis of 5 NuSTAR observations of SGR 1806-20 spread over a
year from April 2015 to April 2016, more than 11 years following its Giant
Flare (GF) of 2004. The source spin frequency during the NuSTAR observations
follows a linear trend with a frequency derivative
$\dot{\nu}=(-1.25\pm0.03)\times10^{-12}$ Hz s$^{-1}$, implying a surface dipole
equatorial magnetic field $B\approx7.7\times10^{14}$ G. Thus, SGR 1806-20 has
finally returned to its historical minimum torque level measured between 1993
and 1998. The source showed strong timing noise for at least 12 years starting
in 2000, with $\dot{\nu}$ increasing one order of magnitude between 2005 and
2011, following its 2004 major bursting episode and GF. SGR 1806-20 has not
shown strong transient activity since 2009 and we do not find short bursts in
the NuSTAR data. The pulse profile is complex with a pulsed fraction of
$\sim8\%$ with no indication of energy dependence. The NuSTAR spectra are well
fit with an absorbed blackbody, $kT=0.62\pm0.06$ keV, plus a power-law,
$\Gamma=1.33\pm0.03$. We find no evidence for variability among the 5
observations, indicating that SGR 1806-20 has reached a persistent and
potentially its quiescent X-ray flux level after its 2004 major bursting
episode. Extrapolating the NuSTAR model to lower energies, we find that the
0.5-10 keV flux decay follows an exponential form with a characteristic
timescale $\tau=543\pm75$ days. Interestingly, the NuSTAR flux in this energy
range is a factor of $\sim2$ weaker than the long-term average measured between
1993 and 2003, a behavior also exhibited in SGR $1900+14$. We discuss our
findings in the context of the magnetar model.
