get an equation for the volume of the liquid in the filter with height as the variable.
differentiate volume with respective to the time. then you'll get an expression with h, $\displaystyle \frac{dV}{dt}$ and $\displaystyle \frac{dh}{dt}$ (h is the height).
you know h, $\displaystyle \frac{dV}{dt}$. substitute and find $\displaystyle \frac{dh}{dt}$.