Basically, only when you make it stop mattering. When you are or plan to soon be a (non-living) construct or undead, you lose your Constitution anyway. There are also (generally considered overpowered) options for using a different ability score for HP, like the Faerie Mysteries Initiate feat which makes your HP based on Intelligence. But every creature using it for HP should have good to great Constitution. A character with Con 12 is already what I would consider “low,” to say nothing of scores of 8 or 6.
Basically, Con just gives too much HP over the course of the game to do anything else. On top of that, the classes with the least need for HP, are also heavily SAD (single ability dependent) classes that need nothing after their casting stat, and they have small HD to boot, so even if they could get away with it, there’s no reason for them to.
So the only time you can truly dump it is when you’re not going to have it because you’ll be Con — or using something else to determine HP.