You could expect a dose of prednisone to stay in your system for 16.5 to 22 hours.

The elimination half life of prednisone is around 3 to 4 hours. This is the time it takes for your body to reduce the plasma levels by half. It usually takes around 5.5 half lives for a drug to be completely eliminated from your system.

Other factors that can cause variation to the elimination time include:

1. How much and how often you have taken the drug.
2. Your metabolic rate – a slower metabolism will increase the time a drug remains in your system.
3. Your age and health – older age and poor health will generally increase the time the drug stays in your system.
4. Body mass – generally the bigger you are the longer a drug will remain in your system.

For more information see: https://www.drugs.com/ppa/prednisone.html