The life-time of ion waves in unstable and turbulent plasmas is investigated experimentally. The life-time, a measure of the auto-correlation time of ion waves, is studied by a Boxcar Integrator. In unstable states, life-time strongly depends on the boundary condition of the plasma. In turbulent states, however, life-time is almost constant and independent of the boundary conditions and the electron drift velocity, whereas the turbulence energy increases with the drift velocity. Such a constancy may be expected from the Wiener-Khintchine theorem in noise theory. Several types of interaction between ion waves like oscillatory energy exchange or mode locking are also examined.