A common braising time for oxtails would be on the order of 2-4 hours.

Oxtails are a tough cut of meat, with a lot of connective tissue made of a protein called collagen. It needs to cook in a hot, moist environment in order to hydrate into gelatin.

This process happens over time and temperature. Typically, braising is on the order of 200 degrees F (93 C), give or take, in order to get the expected tender product in the expected time.

If your oxtails have not become tender already in the 5 hours you have already given them, perhaps your slow cooker's low setting is not hot enough. This is of special concern because you also want it to be at least 140 F (60 C) for safety.

I strongly urge you to use an instant read thermometer and measure the temperature at several places within the pot. If it is not above the safety threshold, you should discard the contents, which would have been essentially incubating for the entire time.

If it is lower than the desired braising temperature, you can try your high setting, or transfer your braise to the oven (if the crock is oven safe, or in another covered oven safe pot such as a dutch oven). Oven braising is usually done at about 350 F (180 C), which should bring the contents up to the desired braising temperature.

In any case, an expected cook time of 20 hours seems excessive by a factor of about 4 times.

Note also: if your goal is to produce beef stock for the soup rather than tender, delectable meat, you want the temperature up around a slow simmer, which is about 200 F (93 C), although at the end, the flavor will have gone from the meat into the broth. Even for stock making, I cannot imagine a cook time being required in excess of more than 8-12 hours.

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.