When the ball reaches its original position, it will have the same speed (but would be traveling in the opposite direction). So the average speed is
[tex]\dfrac{19.6\,\frac{\mathrm m}{\mathrm s}-19.6\,\frac{\mathrm m}{\mathrm s}}{\Delta t}=0[/tex]
regardless of how long the ball was in the air.
