  Spectra of OH/IR stars show prominent spectral bands of crystalline olivine
(Mg$_{(2-2x)}$Fe$_{(2x)}$SiO$_{4}$). To learn more about the timescale of the
outflows of OH/IR stars, we study the spectral band of crystalline olivine at
69 {\mu}m. The 69 {\mu}m band is of interest because its width and peak
wavelength position are sensitive to the grain temperature and to the exact
composition of the crystalline olivine. With Herschel/PACS, we observed the 69
{\mu}m band in the outflow of 14 OH/IR stars. By comparing the crystalline
olivine features of our sample with those of model spectra, we determined the
size of the outflow and its crystalline olivine abundance.
  The temperature indicated by the observed 69 {\mu}m bands can only be
reproduced by models with a geometrically compact superwind
($R_{\rm{SW}}\lesssim$ 2500 AU = 1400 R$_{*}$).This means that the superwind
started less than 1200 years ago (assuming an outflow velocity of 10 km/s). The
small amount of mass lost in one superwind and the high progenitor mass of the
OH/IR stars introduce a mass loss and thus evolutionary problem for these
objects, which has not yet been understood.
