Dx'd with allergic reaction by leg incision 4 wk postop. Rash - red, raised, dry, itchy. Steroid cream 2x day, covered leg in brace. How long last?

Depends. Depends on what type of allergic reaction you are encountering and whether you are still exposed. To the allergen. I presume your problem is from something you came into contact with during surgery and that it is most a skin rash. If no further contact, i'd expect the rash to subside in about a week. If you want a better answer, you will need to give us a better description.