It depends on the sensitivity of the exposed. ex. some won't get it at all. Some will almost immediately feel the itch - see the redness and the rash is quick to follow. I have experienced this within hours or if not overnight. Hope you get better answers than this. Time to get out the calamine lotion.
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them