When people talk about how long dough can sit in the fridge what happens if it sits too long? Does it go bad? Does it get hard? How would I know if I leave it sit too long?
Eventually all doughs will become unusable. When the dough becomes unusable will depend on the type of dough and the dough formulation, especially the amount of yeast, the mode of fermentation (e.g., at room temperature, in the refrigerator, or some combination of both), and the temperature of the dough as it ferments. Sometimes there are visual cues that the dough is about to become unusable, such as a very puffy and soft dough with a profusion of fermentation bubbles and with little or no resistance to the touch, or possibly a slight darkening of the dough with spotting, but sometimes there are few visual cues. Usually the dough becomes unusable when it overferments. That typically happens when the yeast runs out of food (natural or added sugars). Also, with a long fermentation, there are enzymes in the dough, called protease enzymes, that attack the gluten structure over time, causing the gluten to become degraded to the point where the dough becomes highly extensible. When this happens, the water bond with protein is broken, releasing water into the dough, making it wet or slack or "clammy". Such a dough can become hard to handle and it is not uncommon for the dough to develop tears when trying to form it into a skin. Attempts to re-knead or re-ball the dough to restore its physical integrity and character will usually be fruitless, and you are likely to end up with a dough ball that is overly elastic and almost impossible to open up to form a skin. If one is able to actually form a skin out of the dough and to make a pizza out of it, the finished crust is likely to be light in color, because of insufficient residual sugars to contribute to crust coloration, and on the cracker-y side with sub-par oven spring (you need the proper combination of acids in the dough and residual sugar to get good oven spring). The crust is likely to have good flavor, however, because of all of the byproducts of long fermentation that contribute to good crust flavor, as well as aroma.
I often suggest that newbies intentionally let a dough go downhill so that they can see what the phenomena are that are at play as a dough ferments over a long period of time. A good experiment is to make several dough balls and use them one at a time, daily, until they run out, and observe the differences from one pizza to another. For such an experiment to be useful, it is best to make the pizzas identically as much as possible, including the types and amounts of sauce, cheeses and toppings. Otherwise, there will be too many variables to be able to make meaningful comparisons. You might even let the last dough ball expire on its own so that you can see what a dying dough looks and behaves like. Sometimes, people use a dying dough to make breadsticks as not to let the dough to go to waste.