  The very late thermal pulse (VLTP) affects the evolution of $\sim$20\% of
1--8\,$\mathrm M_\odot$ stars, repeating the last phases of the red giant
within a few years and leading to the formation of a new, but hydrogen-poor
nebula within the old planetary nebula (PN). The strong dust formation in the
latter obscures the optical and near-infrared radiation of the star. We aimed
to determine the reheating timescale of the central star in Sakurai's object,
which is an important constraint for the poorly understood VLTP evolution. We
observed the radio continuum emission of Sakurai's object for almost 20 years
from 2004 to 2023. Continuous, multi-frequency observations proved to be
essential to distinguish between phases dominated by photoionization and shock
ionization. The flux density fluctuates by more than a factor 40 within months
to years. The spectral index remained negative between 2006 and 2017 and is
close to zero since 2019. The emission region is barely resolved since 2021.
Non-thermal radio emission observed from 2004 to 2017 traces shocks induced by
wind interactions due to discrete mass-loss events. Thermal emission dominates
during the period 2019--2023 and may indicate photoionization of the nebula by
the central star.
