If a caster has spent the duration of the Levitate spell moving upward to 2000' how long would it take to float back down again?
The levitate spell states:
When the spell ends, the target floats gently to the ground if it is still aloft.
This is all we know about the spell ending, it does not induce feather fall or anything of the sort. We also do not know how fast a gentle float is, so you are going to have to ask the GM (or make a ruling yourself).