  AT 2000ch is a highly variable massive star and supernova imposter in NGC
3432 first detected in 2000. It is similar and often compared to SN 2009ip, and
it is therefore expected to undergo a core-collapse supernova -- a SN imposter
of similar brightness -- in the near future. We characterize the long-term
variability of AT 2000ch in the radio and optical regimes with archival data
reaching back to the year 1984. We use these newly reduced observations in
addition to observations in the literature to restrict the mass-loss rates of
AT 2000ch at multiple epochs based on different approaches, and to infer the
general properties of its circumstellar nebula with respect to the detected
radio brightness. We extend the known optical light curve of AT 2000ch up to
the beginning of 2022 by performing point spread function photometry on
archival data from the Palomar Transient Factory and the Zwicky Transient
Facility. We reduced archival radio continuum observations obtained with the
Very Large Array using standard calibration and imaging methods and
complemented these with pre-reduced \changes observations as well as
observations obtained with the Westerbork Synthesis Radio Telescope and LOw
Frequency ARray. For the analysis of AT 2000ch, we consider the optical light
curve and color evolution, its radio continuum brightness at different
frequencies and times, and the corresponding spectral indices. We estimated
mass-loss rates and optical depths based on radio continuum brightnesses and Ha
fluxes. We report two newly detected outbursts of AT 2000ch similar to those
found in the 2000s and 13 re-brightening events, of which at least four are not
conclusively detected because of insufficient sampling of the light curve. The
dates of all outbursts and significant, well-sampled re-brightening events are
consistent with a period of $\sim 201 \pm 12\,$days over a total time-span of
two decades. Such a behavior has never been found for any SN imposter,
especially not for candidate SN~2009ip analogs. During 2010 to 2012 and 2014 to
2018, we only have a few detections, which is insufficient to come to any
conclusion as to a possible less eruptive phase of the transient. We find
steady dimming after the most recent re-brightening events and possible
evidence of porosity in the circumstellar envelope, suggesting AT~2000ch may
currently be in transition to a state of relative calm. We identified a second,
unrelated source at a projected distance of $\sim 23\,$pc
($\sim0.5^{\prime\prime}$) that has contaminated the optical measurements of
AT~2000ch at its minimum luminosity over the last two decades probably on a
$5\%-10\,\%$ level, but this does not affect our overall findings and is
negligible during re-brightening. We are able to restrict the mass-loss rate of
AT~2000ch to range between several $10^{-6}\,\textrm{M}_{\odot}/\textrm{yr}$
and several $10^{-5}\,\textrm{M}_{\odot}/\textrm{yr}$. The fresh ejecta appear
to be optically thick to radio continuum emission at least within the first
$\sim 25\,$days after significant re-brightening.
