distance fallen = vi*t + 1/2 a*t^2
where vi = initial velocity in the direction that the projectile is falling (0 if the projectile was shot perpendicular to your surface)
a = 9.8 m/sec^2 assuming that the force of earth's gravity is the only thing acting on it. If the force acting on it is not constant you'll have to do some calculus.
t = time