It all really depends. I have a friend that have been stabbed in the thigh and still ran 4 miles home. Two other friends were stabbed in the stomach and back, respectively.
The one who got it in the leg was hit HARD with a 6 inch knife. The blade was removed, he bled like mad but adrenaline was pumping and he made it out of there.
As for the others, one got hit in the lower right side under the ribs and upper back, the other in the sternum and on the left flank. The blades used on them were 3 - 4 inches long. These happened when they were grappling with each other. They had no effect on how the fight progressed, and neither knew they were stabbed until after.
I had another friend who just passed a few months ago, stabbed outside a night club. He didn't make it. The attacker used a 5-6 inch knife and stabbed him multiple times in the chest, sternum, and collar area.
Point is, striking at the legs is a hard target and not always a sure bet to end a fight.
If you're going to stab someone in the chest, use a big knife. 3-4 inch blades won't do the job. 5 is alright , 6 is adequate, 7 is plenty. You want to damage internal organs. That's the target. Slashing around will cause blood to flow but won't immediately stop a fight.
my .02