### The *Player's Handbook* gives no guidance.

The [basic rules for falling][1] say:

>A fall from a great height is one of the most common hazards facing an adventurer. At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall.

### *Xanathar's Guide to Everything* has the answer, and it's probably up to the DM.

In the [**optional** rules for falling][2] in *Xanathar's Guide*, we see:

>The rule for falling assumes that a creature *immediately drops the entire distance when it falls*.<sup>1</sup>

Notably, this rule *is optional*, so it is between you and your DM to establish which rules you are using. Further, if you decide to use this rule, then it is still up to the DM to determine when "falling" really begins. Whenever "falling" begins, the optional rules here say that you immediately drop up to 500 feet. 

I would have ruled that you fell the full distance before the end of your turn. I would make the argument that the falling began at the end of your attack action, and that you dropped the whole distance immediately. But your DM may rule differently, and you should discuss this with them. 

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<sup>1</sup>Note, when it says "the rule" it is referring to the rule for falling from the *Player's Handbook* cited in the first section of this answer. 

[1]:https://www.dndbeyond.com/sources/basic-rules/adventuring#Falling
[2]:https://www.dndbeyond.com/sources/xgte/dungeon-masters-tools#Falling