If you found the vertical velocity, then use that to calculate how long it will be in the air.

Do you know how to solve the problem when the rock is thrown vertically up in the air?

Let the vertical velocity be "v" then the amount of time it will take for the speed of the rock to go to zero would be v/g where g is the acceleration due to gravity (essentially \frac{\text{change in speed}}{\text{acceleration}} = \frac{0 - v}{-g} )

This is the same amount of time, it will take for the ball to come down to the ground. So, the total time it will stay up in the air will be \frac{2v}{g}.

In your particular case, the vertical velocity is given 25 m/s. So, the amount of time it will stay up in the air is 2*25/10 = 5s (assuming the acceleration due to gravity is 10 m/s^{2}).