  Quantum Zeno effect is conventionally interpreted by the assumption of the
wave-packet collapse, in which does not involve the duration of measurement.
However, we predict duration $\tau_m$ of each measurement will appear in
quantum Zeno effect by a dynamical approach. Moreover, there exists a
model-free critical measurement time, which quantum Zeno effect does not occur
when $\tau_m$ takes some special values. In order to give these predictions, we
first present a description of quantum Zeno effect in the Heisenberg picture,
which is based on the expectation value of an observable and its fluctuation.
Then we present a general proof for quantum Zeno effect in the Heisenberg
picture, which is independent of the concrete systems. Finally, we calculate
the average population and relative fluctuation after $N$ successive
measurements in XX model, which agrees with our prediction about the critical
measurement time.
