  The IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX)
mission samples interstellar neutral (ISN) helium atoms penetrating the
heliosphere from the very local interstellar medium (VLISM). In this study, we
analyze the IBEX-Lo ISN helium observations covering a complete solar cycle,
from 2009 through 2020 using a comprehensive uncertainty analysis including
statistical and systematic sources.W e employ the Warsaw Test Particle Model to
simulate ISN helium fluxes at IBEX, which are subsequently compared with the
observed count rate in the three lowest energy steps of IBEX-Lo. The $\chi^2$
analysis shows that the ISN helium flows from ecliptic
$(\lambda,\beta)=(255.59^{\circ}\pm0.23^{\circ}, 5.14^{\circ}\pm0.08^{\circ})$,
with speed $v_\text{HP}=25.86\pm0.21$ km s$^{-1}$ and temperature
$T_\text{HP}=7450\pm140$ K at the heliopause. Accounting for gravitational
attraction and elastic collisions, the ISN helium speed and temperature in the
pristine VLISM far from the heliopause are $v_\text{VLISM}=25.9$ km s$^{-1}$
and $T_\text{VLISM}=6150$ K, respectively. The time evolution of the ISN helium
fluxes at 1 au over 12 years suggests significant changes in the IBEX-Lo
detection efficiency, higher ionization rates of ISN helium atoms in the
heliosphere than assumed in the model, or an additional unaccounted signal
source in the analyzed observations. Nevertheless, we do not find any
indication of the evolution of the derived parameters of ISN helium over the
period analyzed. Finally, we argue that the continued operation of IBEX-Lo to
overlap with the Interstellar Mapping and Acceleration Probe (IMAP) will be
pivotal in tracking possible physical changes in the VLISM.
