Had one last week (already diagnosed with two blocked arteries in my ticker - been there done the angioplasty which was unsuccessful). I was able to walk/run on the treadmill for 8 minutes before I had to stop (chest pain, out of breath, seeing red spots in front of my eyes, systolic blood pressure over 250).
Am I a completely out-of-shape slug for lasting only 8 minutes?
You sound pretty out of shape to me, but I’ve never done one of those, so I have no comparison point.
I’m a reasonably typical tubby American carrying about an extra 30 pounds of avoirdupois, but I can (and do) walk for hours at a time at a sedate pace. As you do the stress test, they increase the speed and the angle of the treadmill, so I was popping quite a few calories by the time I had to quit.
I had one a few months ago. I have aortic stenosis, and will have the valve replaced sometime this year. I don’t know how long I lasted, but my legs gave out sooner than my heart. It got to the point that I was having so much leg pain that I couldn’t continue.
Are they going to install a pig valve or an artificial one? One of my friends had the pork variety put in 15 years ago, and it is still going strong. His quality of life is 180 degrees from what it used to be - the only down side is that he has to take warfarin (blood thinner) every day.
I’ve done two and both times they told me to stop around the 10 minute mark. I think that was all they needed. I could have continued. The first one they stopped it when I took my hands off the rails, it is a treadmill so I don’t know why they expected me to run while holding on. The next time the guy just said you’re done.
I’ve never done a stress test, but I used to work in an MDU. During my training period I got to hang out in each department for a couple of hours and see what they did. They did their tests with the bicycle where the tension gets ramped up every minute. While I was at the exercise test area, there was a 23 year old (young?) man who did 29 minutes. I was mightily impressed. Then the tech told us that the record there was 43 (!) minutes - that guy was a 30 year old firefighter.
I did one once, and that was when I was running 25-35 miles per week. And it didn’t last long until they declared it done.
It seems that they just kept increasing the incline until it was just as much stress on my heart as it would have been for anyone else. I know nothing about the details, but it did seem like they dial in what each person can bear. This would skew any “I lasted x number of minutes” comparison.
The main criterion for fittness for duty for firefighters in this area is the bruce protocol. It’s a cardiac stress test. My husband can do 15 mets easily. I cannot. 
One year ago, he couldn’t do 11 mets, which is the minimum.