|I just read the same thing for poison ivy: 12-48 hours for the rash to develop.
From the FAQ on this site:
If you don't cleanse quickly enough, or your skin is so sensitive that cleansing didn't help, redness and swelling will appear in about 12 to 48 hours. Blisters and itching will follow. For those rare people who react after their very first exposure, the rash appears after seven to 10 days.
New lesions that appear a few days after the primary lesions represent less sensitive areas or areas where less antigen was deposited, not spreading of the antigen.