The length of time that prednisone can stay in your system is dependent on several factors. First, consider the amount of time the drug has been taken and how often it is taken. Next your metabolic rate has to taken into account. A slower metabolism naturally will take longer to dissipate the drug than a faster one. Also you age and health play a part in this determination. Additionally, the larger you are the longer the drug is likely to stay in your system. So, on the average, it takes anywhere from 18 hours to 24 hours to rid the drug from your system.