Withdrawal periods reflect the amount of time necessary for an animal to metabolize an administered product and the amount of time necessary for the product concentration level in the tissues to decrease to a safe, acceptable level. Every federally approved drug or animal health product has a withdrawal period printed on the product label or package insert. Products carry meat withdrawal periods ranging from 0 to 60 days. Examples for meat range from no withdrawal period with ceftiofur, 4-15 days with different penicillin products, to 28 days with Pirlimycin. Animals treated with a product that has a withdrawal period of 45 days should be withheld from sale or slaughter for at least 45 days.
Withdrawal times are not the same for all drugs. Examples for milk include: Pirlimycin, 36 hours; Cloxacillin, 48 Hours, Amoxicillin, 60 hours, Penicillin, 72 hours, and Cephapirin, 96 hours. Milk produced during that period must be disposed of. Dairy animals to be slaughtered must be withheld for the meat withdrawal period specified.
Withdrawal periods may be extended when combinations of drugs are used or when drugs are used in an extra-label manner. In these situations or at any time a producer is uncertain of a specific drug withdrawal period, a veterinarian should be consulted.
Source:
http://www.ext.vt.edu/pubs/dairy/404-403/404-403.html