Bear with me here,
My roommate insist that she wants to keep her spoiled 2% milk in the fridge, it's been 1 month! That thing look disgusting. To my knowledge, you don't use rotten milk for anything/ingredients. So before I throw it out, is there something I don't know of that people use rotten milk for?
The only thing that is even remotely possible in my mind is to use it akin to buttermilk or sour cream (which are intentionally fermented products) in baking.
However, since the culture that fermented the milk was uncontrolled, I would not do so. I recommend discarding.
Soured milk stays good for a long time (similarly to cheeses) - pretty much until mold starts forming. It is a common drink in Eastern Europe, and production is very simple - essentially "happens by itself": if you leave fresh, non-boiled, non-pasteurized milk in room temperature for a couple days, it turns into soured milk. It's used as ingredient for a few pastries, but the primary use is to drink it straight (after mixing the two fractions), e.g. as a drink served along with fried potatoes. Despite what English Wiki says about culture-farming etc, soured milk as a traditional drink really doesn't take any effort to make.
Now, if the milk was boiled, or UHT, or something like that, what you get is curdled milk. This is spoiled, from moment one, and the only practical use is production of casein-based homemade plastic. No kitchen uses at all.
The way to tell one from another - soured milk forms a pretty much uniform mass similar to thick yoghurt, or large chunks, with a layer of transparent whey on the bottom. Curdled milk forms tiny grainy structures, sometimes slightly elongated or curly. They differ in smell significantly too - soured milk has a clearly sour, rather nice aroma similar to kefir, while curdled milk has a definitely unpleasant, dull smell.
I don't know that I'd keep it around for a month. Once it smells off, but before it's gotten chunky, it's fine to use in baking.
Growing up, once a container started to smell off, we'd draw a 'Mr. Yuck' face on the side of the container, and then use it that coming weekend for pancakes or other baked goods.
Once it gets to the point where you can actually see that it's gone off, because it's separating like cheese, I'm not aware of any uses for it.
One month on, it's almost certainly trash.
Three weeks ago... well, it still depends. Was this standard-issue-homogenized-pasteurized milk? In that case, it was trash the minute it started tasting "off".
If it wasn't homogenized, then there was a window in there where you could have put it out to "sleep" at room temperature. It would have separated into a white creamy part and a clear-yellowish watery part. At that point, it would be sour milk, which you could use in recipes that call for buttermilk, or if you're like my mother, drink as-is. If you're like me, you would think it utterly disgusting, but then buttermilk is nasty, too.
As long as the milk isn't moldy or too foul smelling, sour milk is perfect for cheese making (as cheese is just spoiled or sour milk that is curdled. Curds make the cheese and the whey used in lots of other recipes). Simplest cheese to make is to slowly heat up the soured milk in a clean pot til it just starts to steam (not boiling). Then add about 1-2 tablespoons of distilled white vinegar (or lemon or lime juice), and a tablespoon of coarse salt to the drained curds. Put in a cheese cloth and gently wring out the whey and add salt to taste, herbs, or other seasonings. I used about 1/3 gallon of two month old whole milk (kept in colder than normal fridge the whole time), and made enough Ricotta style cheese to make a single serving of pretty tastey homemade ravioli. The whey I probably wouldn't save from milk that old unless I knew more about how to use whey. Any form of milk with less milkfat than whole milk (2%, 1%, skim, nonfat) are in my mind useless for both cooking and drinking as it is mostly water and a tiny amount of milkfat.
