No one knows exactly how long you are immune for after recovering from COVID-19. Experts have agreed that it is at least 90 days. After recovering, you still have the antibodies in your blood from when you were sick. New ones are not created and the old ones gradually disappear from your body.
If you had COVID-19 recently and want to help the patients who have a challenging course of the illness, you can donate plasma. It contains protein antibodies and its donation can help save lives.