# Decoherence time of quantum decoherence and appearance of wavefunction collapse

If decoherence time of quantum decoherence is zero, or if off-diagonal elements of density matrix of quantum system (without environment considered) vanished immediately after interaction with environment, then one can say that "classical" probability transition occurred, and this is why we get appearance of wavefunction collapse and measurement.

However, decoherence time is almost always not zero, and off-diagonal elements of density matrix of system, as far as I know, do not vanish to zero even though they remain very close to zero. If this observation is correct, then I wonder how quantum decoherence explains how/when measurement occurs - with emphasis on "when".