What is the fastest you've ever seen a new coworker quit a job?

Lunch time, but most of the quitters usually make it to at least the whole shift.

before the first day,  right after the 7-day course.  (customer service)

30 minutes                                                

I have seen one that went through the interview via "Teams" process but never shows up on the first day.  Thats pretty fast.

About an hour. 

I worked at a Subways, in college once......and was asked to train the new guy. 

He was told to come in on a Saturday night, and help with the weekend crowd. 

First words out of his mouth were,  "Oh, does it have to be Saturday ? I like my weekends free"- 

Told that weekends would absolutely be required, he was none too pleased, but reluctantly agreed. 

Saturday night comes,  he strolls in about 20 minutes late,  parks his butt in a chair, and  says he was told by the boss to just "observe". 

about an hour goes by, and he decides he's had enough of that....and starts helping me, and making a sandwich.......which I assumed was  for one of the customers. 

He wraps it up,  puts it in a bag, and then grabs his coat and walks out the door. 

Never saw the dude again .

Does it count if they fell asleep during orientation?

They didn't quit voluntarily, but left as fast as HR could process the paperwork.

I saw a guy being shown around to be the new safety manager, walk out during the middle of his first plant tour, he hadn't even been offerfed the job yet ( he would have been )

By coffee break, young people don't like construction its to difficult 

A few years ago I was training a newly hired employee. It was her first day and she had been signing some paperwork for HR and stuff like that... By the time they passed her off to me it must have been just after 10 or so... We chitchatted as I was showing her the ins and outs of the system we used and she told me that she was a single mom. She asked for recommendations for a place nearby to have lunch and I let her go a bit early so that she would beat the lunch crowd. Let's say she headed out around 11:30 or so... She comes back and says "I just talked to a girlfriend of mine during my break and she says that there's an open position at her company across town, but if I want it, I've got to start today." What could I say? I called the manager over and he called her into the office. Their conversation lasted about 30 seconds. She came back to my work station to get her coat and purse, smiled and waved awkwardly, and was gone. She was employed for approximately 3 hours or so.      

Same day quitting isn't that uncommon.  I've done it myself.