Assume it is pasteurized, sealed and properly refrigerated. Nitrogen back-filled if that is a thing. Does the fat content make a difference?
From my experience, as long as it’s kept cold and not left out on the counter for a long period of time, it should last at least 7 days past the sell-by date printed on its container. I have been known to stretch it out to 10 days without suffering any ill effects. YMMV.
Usually our (pasteurized) milk lasts at best a couple of days past the sell-by date before off-odors begin.
That doesn’t necessarily mean it’s unsafe to drink but we toss it anyway.
It’s 1% milk incidentally.
I have opened full-fat buttermilk that is set to expire today. Would it still be good for making biscuits through the weekend?
My experience with lactose free milk is that it lasts significantly longer that milk with lactose. Online search shows a few sites that say at least an extra week.
Nitrogen back-filling may or may not help, but ultra-pasteurization certainly does.
Ultra-pasteurized shelf-stable milk packaged in a sterile container has a typical unrefrigerated shelf life of six to nine months.
How long is it good after the sealed package is opened?
IMO it starts to smell off before it ever gets near the date printed.
Actually, the National Dairy Council draws a distinction between pasteurized milk, ultra-pasteurized milk, and ultra-high temperature (UHT) milk.
With refrigeration, ultra-pasteurized milk lasts about as long as regular pasteurized milk after it is opened.
There seems to be some confusion amongst the replies about whether the OP meant milk still in the factory sealed container, or milk after the container’s been opened and the milk partly used.
IMO / IME there’s a big difference in shelf (fridge shelf that is) life between those two scenarios.
IME open milk is a week to 10 days before it starts getting iffy and another 3-4 sliding from iffy towards yucky. Sealed milk seems to be good for about 3 weeks pre-opening before it starts to go. But goes quickly once opened.
I also have a pretty robust stomach that deals well with iffy food. Unlike some folks who suffer a lot more from a lot less.
I have an OPEN carton of Horizon Organic Ultra-Pasturized Whole milk in my fridge that has an expiration date of October 15, and it still smells perfectly fine. I don’t see how this is possible, but it is true.
And I have an excruciatingly sensitive nose and as soon as it smells even a little off, I’ll toss it.
I think a lot of it comes down to how effective the initial pasteurization is at killing off the bacteria initially, followed by how you treat it after opening the container.
Every time you open the container, you run the risk of contaminating the contents. So if you open the container and keep your fingers from touching the opening, that’s good. Whereas if you do the clueless teenager thing and drink out of the container and put it back in the fridge, you’ve just added a whole lot of bacteria and it will likely spoil much faster.
Or if you leave it out on the counter overly long and allow it to warm up, you can allow bacteria a chance to multiply quickly and thereby spoil the milk.
The thing to remember on Ultra-Pasteurized is that the best-by date is in reference to when the milk is first opened, so if you just opened that a week ago, it would probably be just fine. What you don’t want to do is get UP milk with a best-by date of October 15th, open it on September 1st and expect it to last 45 days. As @robby noted, once opened it’s good for about as long as your standard pasteurized milk, a week or perhaps a bit more, regardless of best-by date.
When I buy milk I always look for a carton with the expiration date as far in the future as I can find. So I’m guessing I opened this carton of milk probably around October 2nd or 3rd.
I don’t drink milk, so the only time I ever open the carton is to pour a little milk in my coffee. And that isn’t even everyday. So it has spent a long time sitting in the refrigerator tightly closed.
Like you, I buy it because it can sit there for a few months until I finally need it (I buy UP heavy cream for the same reason) and it is therefore always on hand. Once opened I write the opening date on the bottle in Sharpie just to keep track. I don’t think I’ve risked going beyond two weeks, but I have occasionally tossed it out sooner than that due to either actual or psychological off smells. It could be a function of my opening it multiple times when I’m repeating some recipe over a few days, as @robby mentioned.
I do that too, once mentioning it on the Dope in a sarcastic rejoinder to a poster who intimated that such behavior was typical of a certain ethnic group.
But it’s best to let sleeping stereotypes lie. 
My Dad was a milkman for a few years and I can remember everything about the Lorain Creamery. The pasteurization machine had a glass window and the milk cascaded over a series of hot rollers and then over chilled rollers and then into the bottling line. The sound of hundreds of bottles clanking along the conveyor belts was quite loud.
They also homogenized their own milk and everyone referred to it as “homo”.
I just bought a quart of half and half with a best by date next May.
We have learned that the milk from one grocery store in our area typically lasts for a week or more past the pull date, while the milk from another grocery store (unfortunately, the one where we do most of our shopping) barely lasts to the pull date and sometimes does not make it even that long. We surmise that it’s because store B is pretty lax about restocking - I have seen pallets of frozen foods sitting in the aisle waiting to be shelved, and I can imagine they may do the same with dairy products, too. That can’t be good for shelf life. When our kids still lived at home and we went through a gallon of milk or more a week, that wasn’t a problem, but now that it’s just two of us, where we buy milk is more important.
Years ago, Grandma told me that milk shelf life could be extended by mixing into it either baking soda or baking powder (I can’t remember which). Milk never lasted long enough for that to be a problem much back when, but now when it was just her and I (a college student) living there, it could happen.
But I recommended that we use old milk up by baking things with it. She said it worked fine for that, and Grandma was a great baker. I was quite willing to make a trip to buy a new gallon of milk, if it meant the old milk became Grandma’s silk pies or biscuits!