Time goes by thickness in sous vide. The minimum is about 1 hour per inch of thickness to get it up to temperature. After that, the rest of the time determines tenderness. If it's a very tender cut of meat, the minimum time will be all you need. But for a large piece with lots of different muscles and tendons, you could need double or triple or more that minimum time to make it tender. That's why you see different times. So if you roast is 6 inches thick, it would take 6 hours just to heat up,, but 12-18 hours to get tender enough for your satisfaction.