When is a regurge an official regurge versus a spit out? Like, is half a mouse down but then out a regurge?
There is a difference between a regurge and "spitting out." If the snake starts to eat the mouse and gets startled by anything and they quickly "back up" and spit out the mouse, that is not a regurge, it is simply spitting it out to make a quick get away from the "trouble."
A regurge is when the prey is swallowed into the stomach and is expelled sometime later, either as a basically whole mouse still or as a partially/mostly digested one.
Also, do regurges usually happen fairly soon after eating, or does it vary? I assume, the longer its been (and the farther down the mouse has gone) the harsher it is on the snake.
I have seen regurges take place shortly after a feeding up to a couple days after a feeding. I have even saw a regurge take place three days later, but this and the ones that usually happen the same day, have all been with snakes that were non feeders that were being force fed and were not in "top" health due to their reluctance to eat. I have only had one adult snake regurge on me, and that was way back when I first got into corns and got an adult female that wasn't in the best of shape and she regurged. Otherwise all of my regurges have been basically babies that were non feeders that I was working with. But others may have noticed other time limits and such.
As far as time spent in the stomach before the regurge, I, personally, haven't noticed any difference between one that regurged the same day and one that did so after two days. Except, of course, that the regurged item was much more digested on the second day. Either way/time, the snake not only regurges the food item but also bacteria from the stomach. It is the important fluids from the stomach that is lost with the regurge that causes a lot of the trouble.
I'm trying to imagine the mechanics of it, as they certainly don't have projectile vomiting like my kids do
Can't say as I have ever heard of projectile vomiting in snakes. You can just imagine how long and all it takes for the snake to work that up from its stomach. Definitley doesn't sound like fun.