  Context: A large fraction of otherwise similar asymptotic giant branch stars
(AGB) do not show OH maser emission. As shown recently, a restricted lifetime
may give a natural explanation as to why only part of any sample emits maser
emission at a given epoch.
  Aims: We wish to probe the lifetime of 1612 MHz OH masers in circumstellar
shells of AGB stars.
  Methods: We reobserved a sample of OH/IR stars discovered more than 28 years
ago to determine the number of stars that may have since lost their masers.
  Results: We redetected all 114 OH masers. The minimum lifetime inferred is
2800 years (1 sigma). This maser lifetime applies to AGB stars with strong mass
loss leading to very red infrared colors. The velocities and mean flux density
levels have not changed since their discovery. As the minimum lifetime is of
the same order as the wind crossing time, strong variations in the mass-loss
process affecting the excitation conditions on timescales of ~3000 years or
less are unlikely.
  Keywords: OH masers -- Stars: AGB and post-AGB -- circumstellar matter
