How long it will last is very variable, based on a lot of factors including your own taste. I get milk from a farm that lasts me about two weeks until it's too sour for my taste. But, some friends don't like it that sour. Another farm, their milk tastes too sour to me after about eight days. It will start out tasting different than you're used to - when it's fresh, it'll be richer and less burnt tasting. It will start to sour slightly after a couple of days. Again, how sour is "too sour" is a matter of your taste.
As for butter, if it's well washed, it'll last for several weeks. If you salt it, it'll last even longer. It'll continue to sour a bit, just like the milk. My dh doesn't like it when it sours, so we try to use it within two or three weeks. But, I've had raw butter for two months that didn't go moldy or anything.