How long does an allergic reaction rash from echinacea last?

A few days. Rashes from something you ingest typically last for just a few days or less. The bigger question is what really caused the rash. People break out in rashes from time to time, and it's just human nature to blame the rash on something we ate in the last few hours. If the rash is continuing for more than a week, it probably has nothing to do with the echinacea; you should see your pcp to check it out.