Interestingly, we had a discussion about this in the MAG-40 class I took last month. Our group included an ER doctor and a few ER nurses.
It seems that the specific chamber that is hit, and whether it is hit during systolic or diastolic pulse, makes a big difference.
Example given was if the heart is nicked while it's diastolic (expanded), then when it squeezes to pump blood, the hole will contract down and mostly squeeze shut (depending on size of the hole); but if the same size hole is cut when the heart is systolic (contracted) it will open up very large during the diastolic.
So the bleedout times can be radically different, even in two people in equivalent health and with equivalent mindset.