If the artery were severed, blood would flow out unimpeded, although the artery wall would contract in an effort to stop the bleeding.
After losing >30% of one's blood volume blood pressure would start dropping, and with less pressure the rate of bleeding would go down.
At this stage if the blood loss wasn't replaced the person could die.
Losing halve to two thirds of one's blood volume is considered to be fatal even if later on blood transfusion is attempted. One's total blood volume at 70ml/kg is estimated to be between 5 to 7 liters, so that makes a blood loss of between 2,5 to 4,7 L.