It's not so much an equation as a method.

Set up the following coordinate system:

Place an origin where the football starts and let the +x direction be in the horizontal direction of the ball and +y be straight up.

Presuming the football starts on the ground and ends on the ground we have the following information:

and

and

and

and

We want an equation for t. It looks like

will do the trick:

<-- Divide through by t

-Dan