For the most part, mass produced milks in America are simply pasteurized, and the most common form is HTST (high temperature, short time) where the milk is heated to a particular temperature (I think around 175 F) for about 15 sec or so. It's not as kind to the milk as vat pasteurization or LTLT (low temperature, long time) which only heats the milk to around 165 for a few minutes. The problem is that it is more of a specialty product, which means 1) it is more expensive, 2) it is hard to find, and 3) it is not a fast-moving product, so it may have been sitting on the shelf for quite a while, which I think is the worst of the 3.
Once I tried a store-bought milk that was low temp, vat pasteurized and non-homogenized. It sounded like the holy grail of supermarket milk. It felt like it too, I had to drive a few hours and to quit a few different stores looking for it. I was sorely disappointed when I got it home and tried to make cheese out of it. It would be an awesome milk to use, despite its high cost ($5 per 1/2 gal) but the cream on top, what little there was, was congealed so hard that it was impossible to mix back in. Additionally, the curd did not set at all the first round, so I had to rennet a second time. When I looked at the carton, it was only 1 day from the sell by date. It tasted old also
There just isn't a lot of turnover for specialty products like that, which means they sit on the shelf far too long to make good cheese.
Regular milk brands have a much higher turnover, so it is easier to find fresher gallons of these than it is for the specialty milks, though I always check the expiration dates of most of the milk types in the store (I spend quite a while in the dairy section). The furtherst sell-by date I have found is 15 days later, so I figure that is about the shelf life of supermarket milk. I'll barely consider milk that is 10 days or less from the sell-by date.