The oxycodone portion of percocet has a half-life of 3.2 hours for the regular version and 4.5 hours for the extended release version. (The half-life of a medication is the time it takes for your body to metabolize and excrete a drug so that half of the drug is removed from your body.) As a general rule you are considered fully clean in blood and urine after 5 half lives. So for regular percocet, that would be 16 hrs. and 22.5 for extended release.
Keep in mind that the half of every drug varies greatly. Half-life can also vary from patient to patient if the patient suffers from altered kidney or liver function. For a more accurate estimate for a specific medication, check with your pharmacist.