As other people have pointed out, per RAW, there are no set rules for actual fall speed outside of certain spells which are meant to slow your fall speed.
Also, something I don't think anyone else has picked up on yet:
At the end of the fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell... (PHB 183)
This actually would imply that it's not the fall that kills you, it's stopping it. So if a player stops their fall instantaneously, you'd better believe that the damage rules still apply to them. Anyway, since the actual falling bit is what's missing, I'm going to derive some rules from real-world physics.
A few simple mathematics (through the SUVAT equations and assuming gravity is roughly on par with that of the Earth's surface) lead to some helpful results (calculations are using metric, actual rule options are given at the end:
In 6 seconds, a you fall ~180m (580ft.), assuming a gravitational constant of 9.8m/s/s. You also reach a speed of 58.8m/s, which is convenient because it is approximately terminal velocity (actually 56m/s.) Having reached 58.8m/s, you then fall at ~360m (1160ft.) every 6 seconds. However we're not quite done here:
Let us assume that the start of any falling will happen, on average, in the middle of a round, giving the players 3 seconds of falling until they can start taking normal actions. In 3 seconds, you only fall 44.1m (145ft.) and reach a speed of 29.4m/s. Then in the subsequent 6 seconds, you fall 264m (865ft.), reaching terminal velocity 3 seconds in. From then on, you fall 360m (1160ft.) per round.
Now then! Let's round out those numbers and tie them into some meaningful rules (we'll round down the later ones since we're actually slightly above terminal velocity):
- In the first round of falling, a falling creature descends 150 feet.
- In the second round, they descend 850 feet.
- In each round after the second, they descend 1150 feet.
These rules assume that if a creature starts falling, it will normally happen roughly in the middle of a round, so they only fall for 3 seconds before they or another creature will be able to use their action to help, rather than assuming that they fall for an entire round. (The downside is that the 20d6 fall damage cap doesn't make sense then, as you fall ~580 feet before reaching terminal velocity, not 200 feet.)
I also did some maths and worked out some physics based off the 20d6 fall damage cap, treating it as if D&D worlds have thicker atmospheres. Works out to a terminal velocity of 34.6m/s, significantly lower than the real world. Ends up a bit simpler, because you actually pretty much reach terminal velocity within 3 seconds.
- In the first round of falling, a falling creature descends 60 feet.
- In each subsequent round, they descend 680 feet.