There are regulations that stipulate a "withdrawal period" so as to allow administered antibiotics to clear from a treated animals system. I am by no means confident that these regulations are always followed, but nevertheless, it is supposed to be practiced.
Antibiotic Residues - A Global Health Hazard Nisha A.R. Assistant Professor, Department of Pharmacology and Toxicology College of Veterinary and Animal Sciences, Pookot, Wayanad, Kerala -673576
The amount of antibiotic residue also varies from animal to animal (pork more than beef for example) and also on an individual animal basis (not all animals receive antibiotics at the feedlot).
You can really get into the subject here... http://www.nap.edu/openbook.php?record_id=21&page=158
Speaking of "excreted", I think most people are unaware that veggies (even organic ones) can contain livestock antibiotics...
"Around 90 percent of these drugs that are administered to animals end up being excreted either as urine or manure," said Holly Dolliver, a member of the Minnesota research team and now a professor of crop and soil sciences at the University of Wisconsin-River Falls. "A vast majority of that manure is then used as an important input for 9.2 million hectares of (U.S.) agricultural land."