How much time does it take for the immune system to go back to its normal functioning state, once corticosteroids therapy stops? (not drug clearance)

Varies: Some decrease in immune system persists after the corticosteroids leave your body, but in clinical practice some patients never lose immunity strength while others develop infections after being on them. In my opinion It's best to assume diminished immunity for weeks after discontinuing those corticosteroids and instruct patients to pay extra attention to any suspicious symptoms during those weeks