R_\mathrm{cl}^{7/6}
n_\mathrm{cl} P^{-5/6}$, where $P$ is the thermal pressure in the hot gas. Since, ignoring the effects of shocks, the density in the clouds is expected to be roughly constant and, as mentioned above, the pressure should decrease as $t^{-6/5}$, the evaporation timescale should increase proportional to $t$. From this we would expect the WL parameter $\tau \equiv t_\mathrm{evap}/t$ to be roughly constant and to only depend on the density and size of the clouds.

For our assumptions regarding mean mass per particle we find $$t_\mathrm{class} = 3.02\times10^{14}\frac{R_\mathrm{cl}^2
    n_\mathrm{cl}}{T_h^{5/2}}\;\mathrm{Myr}$$ and $$t_\mathrm{sat} = 1.48\times10^3\frac{R_\mathrm{cl}^{7/6}
    n_\mathrm{cl}}{P_h^{5/6}}\;\mathrm{Myr}$$ where $R_\mathrm{cl}$ is the cloud radius in pc, $n_\mathrm{cl}$ is the cloud density in cm$^{-3}$, $T_h$ is the temperature of the surrounding hot gas and $P_h$ is the thermal pressure (presumed to be roughly equal in the cloud and hot gas) in cm$^{-3}$ K. We find that with these values, it is not typical to get values for $\tau$ less than one. These expressions also make it clear that the primary determinant of the $\tau$ value is the cloud size distribution. We have confirmed this by doing runs with smaller clouds, using 0.2 and 5.0 pc as the lower and upper size limits, but with the 