Mercutio's wound doesn't seem to be fatal, or at least not as quick a death as it was. The cut is on his lower left side, where there are no organs that are vital enough for their failure to have killed him so fast. And there was not enough blood for him to bleed to death.
This is just a suggestion, but maybe he couldn't take the pain & fainted. Then Romeo & Bendolio might have thought him to be dead, leaving him to die slowly.
Well if you think, in the real book they are sword fighting and Tybalt probably does give Mercutio a fatal wound. For the sake of the book and the movie. he had to die right then or it would have messed up the whole movie.
Actually, you can see an edge of the piece of glass that Tybalt used to stab Mercutio sticking out of the wound. Tybalt probably shoved the piece of glass all the way in Mercutio's side, which would hit a couple of his organs since it was a pretty big piece of glass.
If I am correct, the spleen is on the left side and if the peice of glass the guy got stabbed with was big enough, it could have caused his spleen to rupture...internal bleeding from that kills quickly, but not quickly enough to prevent getting in a few choice words. Willie's contemporaries knew a bit about medicine and (assuming I am right) the educated ones might have known about this type of injury.
Actually, depending on the size of the piece of glass, it could have ruptured his stomach. When the stomach is ruptured, it takes approximately 2-3 minutes (depending on body size - and he was pretty skinny) for the acids in the stomach to poison the body. This is called peritonitis. I know because I had a foreign object in my stomach as a child and had to have emergency surgery to remove it before it punctured my stomach.
3 words-suspension of disbelief.
I'm no medical expert, so I can't comment much on the accuracy of this scene, but either way, this movie isn't exactly the most realistic and scientifically accurate piece of cinema. I also doubt there are many magical potions that allow someone's body organs to stop until they can be confirmed dead, only to wake perfectly unharmed the next day. People in the modern day don't walk around touting Shakespeare, either. But everything is accounted for, and everything is logical, albeit not completely realistic...in a film as fanatsy-like as this, I think that is enough. Sit back and enjoy.