Typically oral steroids
reach their peak effect at ~8 hours after dosing.
This doesn't mean that their work is done. The effect of steroids is cumulative -- daily use for 5-10 days will build on itself, helping more as time goes on. If you're two days into your course, you should begin to see some improvement in the next 24 hours. If not, you may have an infection, and i'd return to the doctor.