If it lasts longer than about 7 or 8 seconds, and the purpose of the attacker is not to take the life, liberty or property of the intended victim, this is a fight. If the purpose of the attacker is to take the life, liberty or property of the intended victim, this is a life attack, regardless of duration. Both situations are dangerous. I've seen guys take shots that you would think would drop a water buffalo, and just keep coming. I've seen a person who had a knife stuck clear through their neck climb up out of a 15 foot ditch into which they'd been thrown, knife still through their neck and their torn scalp falling over their eyes, ready to continue fighting for their life.
Some people are hard to put down and keep down.
The best way to handle a fight is to stay out of it, if possible. A fight makes you tired, gets you dirty, tears your clothes and messes up your hair - and that's if you win. So, while both types of attacks are dangerous, there is a basic difference in the motivation behind them. Most fights are started by bullies. Life attacks are perpetrated by predators.