What angle for firing projectiles gives longest time?

45 degrees to the horizontal will give the maximum flight time for a projectile. If a projectile was fired at 90 degrees to the horizontal, (straight upwards) the projectile will go straight upwards (ignoring the shape, form and aerodynamic properties of the projectile). Likewise if you were to fire a projectile at 0 degrees to the horizontal, the projectile would follow said course, IF gravity was not in effect; a projectile needs some form of vertical velocity to overcome gravity. Hence why 45 degrees will give you the longest distance and consequently flight time.