Percocet is a combination of oxycodone and acetaminophen. The general rule of thumb is that it takes 5 to 6 times the half-life of the drug for it to get out of your system. However, this may vary depending on amount and frequency of use, metabolic rate, body mass, age, overall health, drug tolerance, and urine pH.  Since the half-life of oxycodone is 4.5 hrs., theoretically it should be out of your system in 27 hrs.  I've also seen charts that say it takes 1-5 days for opioids to pass through your system.

Keep in mind, the above estimates relate to urine testing.  Drugs may show up in blood tests for a longer period of time, and in hair analysis tests for as much as 90 days.