The neck shots i have been around were misses, or lapses in concentration. Both younger kids, 12 and 15, Both out of a .243 and both had to be shot in the lungs when we got on them.
old hunting books mention neck shots, but mostly with a 30-06 or a .270. With a larger rifle that has much more energy you could claim the impact of the bullet can rupture the spine without having to break vertebrae. I haven't shot much .223 but I know it doesn't have near the umph of a .270.
if you don't paralyze, they can go a long way before they bleed out. The lungs are still providing air and the heart is still pumping blood. Some of that blood is spilling if you hit a major artery, but it could take hours to be fatal.
hit them in the lungs and they can no longer get air, no longer oxygenate the muscles to run, and lay down. Heart? Game over. Quickly. Every time.