  Solar spots appear to decay linearly proportional to their size. The decay
rate of solar spots is directly related to magnetic diffusivity, which itself
is a key quantity for the length of a magnetic-activity cycle. Is a linear spot
decay also seen on other stars, and is this in agreement with the large range
of solar and stellar activity cycle lengths? We investigate the evolution of
starspots on the rapidly-rotating ($P_{\rm rot}$ $\approx$ 24 d) K0 giant XX
Tri, using consecutive time-series Doppler images. Our aim is to obtain a
well-sampled movie of the stellar surface over many years, and thereby detect
and quantify a starspot decay law for further comparison with the Sun. We
obtained continuous high-resolution and phase-resolved spectroscopy with the
1.2-m robotic STELLA telescope on Tenerife over six years. For each observing
season, we obtained between 5 to 7 independent Doppler images, one per stellar
rotation, making up a total of 36 maps. To quantify starspot area decay and
growth, we match the observed images with simplified spot models based on a
Monte Carlo approach. It is shown that the surface of XX Tri is covered with
large high-latitude and even polar spots and with occasional small equatorial
spots. Just over the course of six years, we see a systematically changing spot
distribution with various timescales and morphology, such as spot fragmentation
and spot merging as well as spot decay and formation. An average linear decay
of $D$ = $-$0.022 $\pm$ 0.002 SH/day is inferred. We found evidence of an
active longitude in phase toward the (unseen) companion star. Furthermore, we
detect a weak solar-like differential rotation with a surface shear of $\alpha$
= 0.016 $\pm$ 0.003. From the decay rate, we determine a turbulent diffusivity
of $\eta_T$ = (6.3 $\pm$ 0.5) $\times$ 10$^{14}$ cm$^2$/s and predict a
magnetic activity cycle of $\approx$ 26 $\pm$ 6 years.
