Being stupid when I was younger, and being in a crapload of them, I would have to say that the circumstances govern what the "unwritten" rules are. If you get into a squabble in the bar over a game of pool with an aquaintance that you get along with sober, I don't think kicking the shit out of him when he's down until he's unconsious will be seen as excusable. But if a street fight breaks out between you and a stranger because you woke up and he was lurking in your house at night would be something I would see as potentially life-threatening, and I wouldn't stop until the other guy was thoroughly beat enough not to be a physical threat any longer.
Also, if a fight between you alone and, say, a drunk radical racist along with three of his buddies will certainly warrant all out tactics like eye gouges and kicks to the balls. They probably won't stop beating on you until you are due a hospital stay, so going all out would be fair.
An ex-boyfriend coming after you, I don't think, would warrant an excessive beating, like kicking the shit out of him after he's knocked out. Putting him in his place to the point of him knowing that any further attempts at fucking with you would be unwise on his part would probably be good enough. A KO, with a weeks worth of a black eye would get the message across well. A husband coming after you because youre messing with his wife, to me wouldn't warrant anything more than dodging and parrying, and minimal effort to really hurt the guy. Stop him, yes, hurt him and give him a beating, nope. If your messing with a married woman, you probably deserve a beating from him in the first place. But to turn it around and beat the living shit out of him and say, break his arm with a keylock, because he's pissed and wanted to punch you, that just doesn't fit the scenario.
There's all kinds of different scenarios, I can't say that I would go "no rules, all out (eye gouges, dirt to the eyes, etc...)" with all of them.
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