homework and exercises - Time to fall in a non-uniform gravitational field

In an uniform gravitational field the loss in potential energy (the work done) by gravity is given by $\text{-}mg\Delta h$ and the time to do so (assuming no friction and no initial velocity) is given my $\sqrt{\frac{2 \Delta h}{a}}$, where $a$ is the strength of the field as commonly referred to the local gravity.

Now, you can do the same for a non-uniform field. To get the work done you simply take the difference in gravitational potential energy: $\int_\infty^{h_1} G\frac{Mm}{{h_1}^2} - \int_\infty^{h_2} G\frac{Mm}{{h_2}^2}$.

And here is my problem. I'm stuck. How does one calculate the time it takes to fall from $h_1$ to $h_2$ with respect to the changing field strength?