How Long Until Hair Grows Back After IPL?

HOPEFULLY NEVER! But there is always some regrowth due to the fact the hair shafts are in differing growth phases and also the chaos theory. From MIAMI Dr. Darryl J. Blinski

Each individual IPL treatment affects only the actively growing hairs at the time of the treatment.  What percentage of hairs are in the actively growing phase varies by body site.  In addition each treatment tends to synchronize more of the hair follicles into the same phase of growth, so each subsequent treatment is more effective.

IPL does not destroy the hairs, it miniaturizes them to such a degree that they are not seen above the surface of the skin.  So the hairs are still there and can potentially start growing again.  How long it would take, if ever, is not known, but for example hormonal changes in women during menopause can cause the treated hair follicles to produce terminal hairs (darker, longer hair) as opposed to vellous ones that are not seen from the surface.