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You start falling when you go prone. ThusThe only part of the RAW that covers this is a very brief section on page 183 of the PHB, from it we can assert this:

  1. She starts falling immediately, on Bob's turn.

  2. She hits the ground immediately as well, on Bob's turn.

There's not a lot to go on here, other than that, falling happens, and that there is no time so it's assumed to happen immediately.

Unlike in previous editions (at least in 4e), 5e contains no provisions for multiple turn falls (though you may rule that certain falls do take place over multiple turns, see addendum).

So basically, you, in an instant, fall all of the distance between where you are and the ground. The only thing that can save you is (As you say) a reaction cast of feather fall or similar.

Addendum, since this is attracting downvotes:

99% of the time, multi-round falling is completely and utterly irrelevant to D&D. For instance, in the example presented in the OP, the Aarokockra is already nearly out of combat effective range, and as part of the example, completely removes herself from it. That's not a useful strategy unless you're running away (you're out of range of a lot of spells, and you're definitely out of effective range for weapons). So for 99% of situations, you can say that the fall is immediate.

The times where this actually does come up is more for mid air battles; something the rules already don't cover well. Thus if you're running a mid air combat, you're already going to need some written house rules in order to run it effectively. Depending on what you expect to happen, you may need to specify how many feet you fall per round. For this you could do the math (remember to factor the proper gravity coefficient for your setting), or borrow a number from a previous edition (4e uses 500' per round). Whatever you choose, it should be more than 60' per round as this is what feather fall uses, and it should likely also be more than 200' per round as this is what you take for falling damage when you hit.

All this to say, if multi-round falling comes up in your game, you're already probably into the domain of house rules with in air combat, so rule accordingly.

You start falling when you go prone. Thus,

  1. She starts falling immediately, on Bob's turn.

  2. She hits the ground immediately as well, on Bob's turn.

Unlike in previous editions (at least in 4e), 5e contains no provisions for multiple turn falls (though you may rule that certain falls do take place over multiple turns).

So basically, you, in an instant, fall all of the distance between where you are and the ground. The only thing that can save you is (As you say) a reaction cast of feather fall or similar.

You start falling when you go prone. The only part of the RAW that covers this is a very brief section on page 183 of the PHB, from it we can assert this:

  1. She starts falling immediately, on Bob's turn.

  2. She hits the ground immediately as well, on Bob's turn.

There's not a lot to go on here, other than that, falling happens, and that there is no time so it's assumed to happen immediately.

Unlike in previous editions (at least in 4e), 5e contains no provisions for multiple turn falls (though you may rule that certain falls do take place over multiple turns, see addendum).

So basically, you, in an instant, fall all of the distance between where you are and the ground. The only thing that can save you is (As you say) a reaction cast of feather fall or similar.

Addendum, since this is attracting downvotes:

99% of the time, multi-round falling is completely and utterly irrelevant to D&D. For instance, in the example presented in the OP, the Aarokockra is already nearly out of combat effective range, and as part of the example, completely removes herself from it. That's not a useful strategy unless you're running away (you're out of range of a lot of spells, and you're definitely out of effective range for weapons). So for 99% of situations, you can say that the fall is immediate.

The times where this actually does come up is more for mid air battles; something the rules already don't cover well. Thus if you're running a mid air combat, you're already going to need some written house rules in order to run it effectively. Depending on what you expect to happen, you may need to specify how many feet you fall per round. For this you could do the math (remember to factor the proper gravity coefficient for your setting), or borrow a number from a previous edition (4e uses 500' per round). Whatever you choose, it should be more than 60' per round as this is what feather fall uses, and it should likely also be more than 200' per round as this is what you take for falling damage when you hit.

All this to say, if multi-round falling comes up in your game, you're already probably into the domain of house rules with in air combat, so rule accordingly.

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You start falling when you go prone. Thus,

  1. She starts falling immediately, on Bob's turn.

  2. She hits the ground immediately as well, on Bob's turn.

Unlike in previous editions (at least in 4e), 5e contains no provisions for multiple turn falls (though you may rule that certain falls do take place over multiple turns).

So basically, you, in an instant, fall all of the distance between where you are and the ground. The only thing that can save you is (As you say) a reaction cast of feather fall or similar.