As this is somewhat my area of expertise, I'll weigh in. Lower abdomen is probably the most likely spot for a slow internal bleed out. (2-8 hours) If the blade misses the vitals and hits the bowels instead, the victim has a good chance of surviving the stab wound only to die of infection, later. (as Kevin pointed out.) Stab wounds to a lung can take a while, but if the wound is deep, the lung can collapse, sometimes against the heart. A shot under the armpoint only has to penetrate (on average) 5 cmm before it hits the outer wall of the lungs. (Air embolisms become a very possibility with this type of wound, in which case death is quick.)
I've been stuck through a shoulder blade. Not much of a chance for death unless the blade is: A. longer than your average 4 1/2" folder, or the attacker is strong enough to bury the blade up to the handle, B. blood pressure bleeds you out, (blood pressure rises like a ***** when you're stuck, even when you're prone) C. The wound channel is excpetionally wide.
For a really slow bleed, the wound channel has to bypass the liver, spleen, kidney, aorta, vena cava, iliac, and mesenteric vessels. That's not as tough as it sounds, provided the entry point is at least a couple of inches below the navel and angled down. Again, your mileage may vary, but you can hang on anywhere from 2-8 hours with a deep, lower ab wound. Longer if the wound misses all vitals. Still have to consider the high possibility of embolisms. Gut wounds are nasty business.
One other note regarding stab wounds to the lower extremities. As someone pointed out, knife attacks frequently turn into wreslting matches, unless the weilder is practiced and proficeint. Most wounds to the legs or lower ab occur when the victim falls to the ground and kicks out at the attacker, or curls into a fetal position for cover. In these cases, the wounds are generally slices and not stabs.
A serial killer who purposely sticks a victim below the navel is either betting that help is a longggg way off and he wants the victim to suffer, or he's begging to be identified by his victim in a line-up at a later date.
Oh, and tell Mr. Serial Killer to stay away from the neck if he wants a slow death. There's very few spots where he can inflict damage that will cause death 2-3 hours later, and a great many spots where he'll kill in seconds or minutes. The neck is first choice for a knifer and a close second for a sticker (ice pick, stiletto, etc.)
Quick kills are much simpler to plot.