Seven days we have to withhold a calf after drinking antibiotic milk (ie, if it's mother was treated for mastitis or if the antibiotic milk accidentally got mixed with the baby calf milk), meaning, the calf has to be drinking clean milk for seven days before it can be sold for slaughter.
If a pregnant cow is treated the meat withholding for the cow also applies to the unborn calf.
Lots of calf rearers here buy waste milk - you want to know why it's 'waste', if a treated cow was accidentally milked into the main tank the whole lot has to be dumped but in reality the antibiotic might not even be detectable... on the other hand, the milk from one treated cow I usually only give to calves four weeks or older, preferably to non-breeders (not replacement heifers).
If you can't use it immediately you need a way to store it/keep it fresh. I've seen trouble with calves being fed from tanks that were chilled and not stirred, they got sick when they got to the cream at the end. I store surplus colostrum unchilled and stirred daily but mix it with good milk immediately before feeding and only to older calves.