Well, this is a simple free-fall problem. We have an initial speed and a vertical distance, so, we use our traditional kinematic formulas to solve the problem:

First, we record what we have:

Now, if there are 2 or less question marks, we can solve the problem, so it looks solvable.

The next part would be to pick the correct equation, A first step would be to find the final velocity, this equation is perfect for that:

Now we plug in what we know:

Now, we use what we have now to find the time, this equation is simple and elegant:

And there you go.