Well I make buttermilk biscuits from scratch using buttermilk (obviously) and I also marinate chicken for frying in buttermilk. I have also found that it keeps quite well past the "best buy" date on the container, especially if you keep it in the coldest part of your refrigerator.
Sorry - typing on my tablet is baaaddd!
1/2 cup. I have a bunch of little tupperware type containers that hold 12 cup. I freeze stuff like butter milk and stock in them, since its a common measurement in recipes.