  In this work we generalize the quantum trajectory (QT) theory from Markovian
to non-Markovian environments. We model the non-Markovian environment by using
a Lorentzian spectral density function with bandwidth ($\Lambda$), and find
perfect "scaling" property with the measurement frequency ($\tau^{-1}$) in
terms of the scaling variable $x=\Lambda\tau$. Our result bridges the gap
between the existing QT theory and the Zeno effect, by rendering them as two
extremes corresponding to $x\to\infty$ and $x\to 0$, respectively. This
$x$-dependent criterion improves the idea of using $\tau$ alone, and
quantitatively identifies the validity condition of the conventional QT theory.
