Some time ago, I did some research on this topic and attempted to explain it in response to a post by a new member.
What I found is that dough actually goes through what I would best describe as a bell shape type of process. Once the dough comes together into a rough dough mass and is kneaded (on the rising part of the bell curve), it will be soft and supple and fairly extensible (stretchy). The more you knead it, the tougher the gluten structure will become and the less supple the dough will become and it will be more elastic. At some point, on the down part of the bell curve, the dough crosses over from being properly kneaded (in the middle of the bell curve) to overkneaded. If you continue to knead beyond the overkneaded point, interestingly the dough starts to get soft and more supple again. That is because the gluten structure is being dismantled by the additional kneading. You might even be fooled into thinking that the dough is OK because it is still soft. With continued kneading beyond this point, the dough can actually go through what is called a “letdown” stage where it completely falls apart and gets very soft and super sticky. At that point, the dough has had it and shouldn't be used. Sometimes the heat of kneading can get so high as to actually kill the yeast (above about 140 degrees F). Then the dough is, in effect, dead and cannot be resurrected.
The best way to see the entire process unfold before your eyes is to use a food processor--as I have done for experimental purposes-- because it does everything at super speed and produces enormous amount of heat.
As you might guess, most pizza operators try to stay on the underkneaded part of the bell curve. It's better for the dough, it saves time, and it saves wear and tear on their mixers.
I realize that the above explanation is not entirely comforting or helpful in telling you where the safe zone is. And, as you know, if your mixer is anything like mine (a standard KitchenAid home mixer), it can produce results that can vary quite significantly from one dough batch to another, even of the same size. And you may also find that a Neapolitan style dough made with, say, a 00 flour, will behave differently from a dough made with a different flour. The best I can offer on this point is the somewhat generic advice given by Tom Lehmann--which I have posted before--on how to tell when the dough is right. It may not be perfect, and he did not render the advice in respect of a Neapolitan dough--and others have even better advice--but FWIW here it is:
You want to mix the dough just enough so that when you take an egg size piece of dough, and form it into a ball, then holding it in two hands, with the thumbs together (pointing away from you), and on top of the dough piece, gently pull the thumbs apart. The dough skin should not tear. If it tears, you should mix the dough a little longer. The dough will have a decidedly satiny appearance. Prior to the satiny appearance the dough will have more of a curdled appearance. Do not stretch the dough out between the fingers to form a gluten film. This test for development is for bread and roll doughs, not pizza. Pizza dough is not fully developed at the mixer, instead, it receives most of its development through biochemical gluten development (fermentation). After the dough has been in the cooler for about 24 hours, you should be able to stretch the dough in your fingers and form a very thin, translucent gluten film.