  We present our past and current long-term monitoring program of water masers
in the circumstellar envelopes of evolved stars, augmented by occasional
interferometric observations. Using as example the Mira-variable U Her, we
identify three types of variability: periodic (following the optical
variation), long-term (years-decades) and short-term irregular (weeks-months).
We show there are regions in the maser shell where excitation conditions are
favourable, which remain stable for many years. Lifetimes of maser clouds in
the wind-acceleration zone are of the order of up to a few years. Much longer
lifetimes are found for the peculiar case of a maser cloud outside that zone
(as in RT Vir), or in some cases where the motion of spectral features can be
followed for the entire 2 decade monitoring period (as in red supergiant VX
Sgr).
