Prednisone clears your system quickly, but how quickly I’m not sure. It has a short 1/2 life. Did you gain much weight or get a ‘puffy’ looking face? Nasty side effects.
Dextramethasone however doesn’t. It has a 36-56 hour half life so it will be at least a week to two weeks. It depends on the dosage though, since it will get to a biologically negligible level quicker if it was a lower dose.