Oxtail soup, what is the normal simmer time?
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.