After [answering this question][1] about how deep can PCs go underwater, I started wondering how long can you keep swimming before you die. Here are the constraints:

 * Your speed is 30 ft and you have no swim speed

 * You are diving straight down, and the water is deep enough to accommodate your travel

 * You are not benefiting from Adv/D.Adv beyond what Exhaustion grants you

 * You die only when you reach an Exhaustion level of 6

This is a probability question, which means there is no 100%. So, to be useful, how long can a creature swim before they have a probability of death equal to 95% or greater?


  [1]: https://rpg.stackexchange.com/a/100015/27327