I want to write a realistic death scene.?

Okay, well first visual things are always good. You could explain how Violet is getting home from wherever and hears her mother scream. Then how Violet can't open the door fast enough before seeing her mother fall to the floor before her very eyes.

If someone were stabbed inches above the navel...I'm not sure I've seen many movies where the people were shot in the stomach and they coughed up blood before dying. I'm not sure it's the same though. Maybe do some research? The how long can be maybe three minutes..depending on your choosing. Violet could be in total shock those three minutes.

What happens would depend on factors such as, is there internal bleeding, are any organs damaged, any blood vessels or nerve endings severed, etc. Personally, I feel this to be a situation where you can more or less use artistic license. What do you see happening if you were to suddenly walk in on a stabbing? Do you imagine a lot of blood? Do you imagine very little blood and very little noise from the victim? i.e. are they screaming, moaning, crying? You don't have to have medical accuracy, necessarily unless you planned on putting in gratuitous details such as what's going on in their body, like the knife collapsed their lung, or penetrated their stomach. And as I just mentioned, I think details like that would be unnecessary and gratuitous for the most part. If you do want to include such details, you really need only a basic grasp of anatomy. Say something like, the knife penetrated so-and-so's side and he/she felt the breath leave them as it entered the lung tissue. lol, this is just an example. It probably sounds stupid and over dramatic (even more so than you want). But you don't have to spout all kinds of medical jargon to make the scene work.

Well, it depends if the knife is pulled back out! If it is then blood would probably spout from the stomach and mouth. It would really flow from the stomach wound. It wouldn't take very long, only a couple of minutes. She would bleed out very quickly. The murderer would have to run out a different way, with his back to Violet the whole way. And then you could be very descriptive in the facial descriptions, etc.

It would probably take a couple minutes for the victim to die, seeing as a rupture of the intestine is not an immediately fatal puncture. The person will die, eventually, but since it will take a while, the bleeding will keep going on as long as the heart is still beating. They will probably loose control of their bowels soon after the stabbing. I don't know enough about wounds to know whether the blood would flow or spray.