I usually ignore the sell by date and give up about a week after opening, as at that point there is usually the beginnings of an off odor. This is with standard store brand skim milk. We usually buy it in quarts. (I eat my morning cereal during the week here at the office, so I'll buy a quart here. Then we buy a quart at home for the weekend.)

There's a possibility different container materials may lengthen the shelf life (the cheapo store brand is in clear plastic jugs, even for the quarts and half-gallons, vs the opaque plastic or the paperboard containers), but it's not worth the price differences or searching around in different stores to find this sort of thing.