Oxtail soup, what is the normal simmer time? I have recently made two very successful attempts at an oxtail soup. 

First boil/simmer:

1.2 kgs oxtail
Yellow onion about equal in volume, maybe slightly less
Carrots about 1/4 the onions, so 2 - 4 carrots depending on size.


I Brown the meat and precook the onions and the carrot. 

My first attempt simmered for about 5 1/2 hours. (The "recipe" called for three) and when I got the tailbones out I had to use my hands a lot to get the meat off of the tails. Digging in good. I had to use a knife to get the gristle off. 

My next attempt I added celery root and tomato puree in the first boil. After  3.5 hours I could easily get the gristle off by just thumbing it off. The meat came off the bone like pork cooked to well done + 5 hours. 

They both tasted awesome, but behaved very differently. This does not seem to be a fire and forget dish. 

Why did they come out so differently?

The soup is completed by using the broth from the first cooking, a good idea is to sift with a cloth. Add the meat, some veggies that you like, celery, mushrooms, carrot, season to taste and you have an amazing soup.
There is no single expected time when you are using agricultural products like oxtail.   Each one will be different.  They may be cut to slightly different sizes.  The animals may have exercised their tails more or less.  One may be younger or older at the time of slaughter.  

Every recipe is a guideline.  You are looking for an outcome, not a duration.  You cook until done, not for a specific duration.

The test for oxtail soup mostly is that the texture of the oxtail has reached your desired level of tenderness.   Then you can stop simmering.

I suspect your two batches straddle the line, though; 3 hours is probably on the order of what you can expect most of the time, and 5 hours much longer than normal.   I surveyed the top google results for "oxtail soup recipe" and most indicate a simmering time of between 2 1/2 and 3 hours, with a couple of outliers.