Xanathar’s Guide to Everything (p. 77) has this to say about the rate of falling:
The rule for falling assumes that a creature immediately drops the entire distance when it falls. But what if a creature is at a high altitude when it falls, perhaps on the back of a griffon or on board an airship? Realistically, a fall from such a height can take more than a few seconds, extending past the end of the turn when the fall occurred. If you'd like high-altitude falls to be properly time-consuming, use the following optional rule.
When you fall from a great height, you instantly descend up to 500 feet. If you’re still falling at the start of your next turn, you descend up to 500 feet at the end of that turn. This process continues until the fall ends, either because you hit the ground or the fall is otherwise halted.
Now, imagine the following “cartoonish” situation : Bob the Unlucky Hiker is climbing a very high mountain and almost reached the top, when an evil Warlock surprises him and releases a Readied Repelling Blast that pushes Bob off the ledge, and he falls 400 feet down to the next “plateau” of the mountain, where he painfully lands (and stops moving). But today is not his day, and another evil Warlock surprises him right after he lands and releases another Readied Repelling Blast that pushes him off that new ledge and he falls for another 400 feet before painfully landing on another ledge. The process repeats itself again and again, because damn, Bob is an Unlucky Hiker.
Does the rate of falling optional rule apply per fall (making Bob suffer several crushing falls in a single turn), or per turn (making the second fall of the above scenario “pause” at 100 feet to continue at the end of Bob’s next turn)?
Best Answer
To each fall, because D&D doesn't do physics well
There is no provision in the rule for multiple falls per turn, so the rule is applied the same to each. The scenario given will (using that optional rule) go as follows:
After the first Warlock initiates the chain, it will follow these looping steps:
Bob starts a (new) fall from one platform.
As per that rule, when he starts falling he descends up to 500 feet.
Bob lands on the platform 400 feet below. The fall is over.
Next Warlock casts eldritch blast and pushes Bob off the platform
This repeats until we run out of Warlocks/Platforms, one of the Warlocks miss, or Bob dies and is a corpse rather than a creature (Repelling Blast requires a creature). At no point does the turn end. This is dumb. It is important to mention that D&D is not a physics simulator and edge cases (hah!) like this are where the DM is expected to use their common sense. A more rigorous rule might be:1
However this rule is also likely to cause some problem in a specifically constructed scenario, though I haven't figured that one out yet.
It's worth emphasizing that this is an optional rule provided as a DM tool (as is my proposed one) and it might be more useful to think of it as how falling could be approximated inside of the initiative system and not a replacement of a DM's judgement.
1: If you think this rule is a bit convoluted, you may have found the reason the rule in XGtE doesn't try to be rigorous.