This is what the spell description say about Levitate ending
When the spell ends, the target floats gently to the ground if it is still aloft.
This would imply that no matter the height, the target would just fall to the ground and take no damage. This could be intended to work similarly to Feather Fall.
Otherwise, if the spell is still active
You can change the target’s altitude by up to 20 feet in either direction on your turn.
Since one turn is ~6 seconds, and the duration for the spell is “Up to 10 minutes”, if you maintain concentration for 10 minutes, you could move your target 2,000 feet up. Given the limitations of the spell, you should at max be able to move a creature 200 ft per minute, regardless of direction.