for opiates like oxycodone it has a half-life of 3.5 to 4 hours ,so from the time you stop you last dose is what you go by. for instance, if I took a oxycodone, I would count the 4 hours( the pills half life), then 3 to 5 days for it to be out of your urine if you are a chronic pill user ,it could take longer.