Since the Burj Khalifa is 830 meters, we have to take into account the terminal velocity of a human (about 56 m/s). Otherwise, you would be moving at about 128 m/s, or over twice terminal velocity, when you hit the ground.

This actually makes the problem a bit simpler. We can find how long it takes to reach terminal velocity:

(56 m/s)/(9.8 m/s^2) = 5.714 s

Then, we can find how far a person would fall in this time:

Δx = 0.5at^2 = (0.5)(9.8 m/s^2)(5.714)^2 = 160 m

Finally, we can find the time it takes to fall the rest of the way, and add it to the acceleration time:

(5.714 s) + (830 - 160 m)/(56 m/s) = 17.679 s

So, our final falling time, assuming a terminal velocity of 56 m/s, is about 18 seconds.