The milk will keep for as long as it had days left before you froze it. If it was going to only last 4 more days when you froze it then it will last 4 days after you freeze it. Makes sense, right?
We've done this when we go to WDW. Freeze our milk before we leave and then when we come home take it out of the freezer at night and put it in the fridge. Ready to use for breakfast the next a.m. And nice and icy cold too!
We've had milk freeze, by accident, at WDW, and it has been kinda nice to hove it so cold.