  General evaluation of the relaxation time to equilibrium is usually
considered as difficult, since it would strongly depend on the model of
interest. In this paper, we provide a generic initial relaxation time of the
fidelity for the isolated large systems. The decay of the fidelity is a
combination of the Lorentzian and a sinusoidal oscillation. We calculate the
relaxation time of the Lorentzian envelop, and the period of the oscillation.
Remarkably, these two time scales are the same order when the energy range of
the microcanonical state is larger than the thermal fluctuation. Also, the
power law decay generally exists for long time regime.
