  We derive a new formula for the longitudinal HBT-radius of the two particle
Bose-Einstein correlation function from a new family of finite and exact,
accelerating solution of relativistic perfect fluid hydrodynamics for a
temperature independent speed of sound. The new result generalizes the
Makhlin-Sinyukov and Herrmann-Bertsch formulae and leads to an advanced
life-time estimate of high energy heavy ion and proton-proton collisions.
