Milk is ok to drink for longer than the frequent recommendation of 7 days after it's opened. Bacteria did not start to proliferate until 13 days after opening during proper (4-degree Celsius) storage. "The total bacterial count in pasteurized milk exhibited accelerated microbial growth from day 13 onwards." This study used PCR methods, much like COVID testing, to look for DNA rates than just trying to grow bacteria in a petri dish. That's important because 99.999% of bacteria are unknown - we know if about 43000 species out of the estimated trillions of bacterial species.
What about the expiration date? Milk can still be ok, even for several days after its expiration date, for several days after opening. Once it's 6 days after expiry, it goes bad within 48 hours after opening - see graph.
Surprisingly, we can safely drink milk with 100,000 bacteria* per ml, or 500,000 in a teaspoon. Above that, it drops to "grade B" milk up until 300,000 bacteria per ml. Safe numbers of coliform bacteria (from poop) are much much lower at 10 per ml.
Not surprising, I guess, when you see the conditions in even the most well-maintained dairy farms. Of course, the cows' teats are very thoroughly sanitized before milking, and even occasionally singed with an open flame to remove hairs that might harbor bacteria.
Also, be sure to store your milk in a part of the fridge that stays cold when you open it, not in the door of the fridge.
*technically, it's "colony-forming units" rather than bacteria, since you can't tell if a spot of growth on a petri dish came from just one bacterium or a clump of many.
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