In the oblique projection a projectile reaches the maximum height in 8 seconds it is time of flight is?

If a projectile takes 8 seconds to reach its maximum height, it will take another 8 seconds to return to its original elevation.

Presuming it is lauched from flat ground and returns to the ground, its total time in flight would be 16 seconds.

If it is launched from a hill, or at a hill, more information would be needed.