Answer
To spend twice the time in air, the ball should be thrown 4 times the height.
Work Step by Step
$v=u+at$, (u being the velocity during throwing up)
$0=u-9.8t\,$
$u=9.8t$
Now,
$S=ut+\frac{1}{2}at^{2}=9.8t^{2}-\frac{1}{2}\times 9.8t^{2}$
$S=4.9t^{2}$
So, to spend twice the time in air, the ball should be thrown 4 times the height.
