It doesn't matter whether you flick a marble horizontally from the roof, fire a high-power rifle horizontally from the roof, drive a school bus straight off the roof, or drop a bowling ball from the roof with zero horizontal speed. Their vertical speed is completely determined by gravity, (and it happens to be the same for all of them).
Handy dandy formula for the distance covered by anything that starts out with zero speed and accelerates to the end:
Distance = (1/2) (acceleration) x (time)²
If the beginning of the journey is on Earth, then the acceleration is 9.8 m/s² ... the acceleration of gravity on Earth. We'll assume that the 55-meter rooftop in the question is part of a building on Earth.
55 meters = (1/2) (9.8 m/s²) x (time)²
Divide each side by 4.9 m/s² : 55 m / 4.9 m/s² = (time)²
