  When particles/molecules diffuse in systems that contain obstacles, the
steady-state regime (during which the mean-square displacement scales linearly
with time, $\left< r^2 \right> \sim t$) is preceded by a transient regime. It
is common to characterize this transient regime using the concept of anomalous
(sub)diffusion with the scaling law $\left< r^2 \right> \sim t^\alpha$, where
the corresponding exponent $\alpha<1$. We propose a new method to estimate the
critical time $t^*$ that marks the transition between these two regimes. The
method uses short-time data from the transient regime to estimate $t^*$, which
can then be used to estimate the steady-state diffusion coefficient $D$. In
other words, we propose a procedure that makes it possible to estimate the
steady state diffusion coefficient without reaching the steady-state. We test
the procedure with various two-dimensional lattice systems.
