"The rash can last anywhere from several days to several months, even over a year. It can come and go in cycles that are related to the treatment the Lyme’s patient is receiving. Treatment often causes something called the Herxheimer reaction, a presentation or relapse of symptoms."
http://www.lyme-disease-research-database.com/lyme-disease-rash.html
This article is more conservative:
http://www.ncbi.nlm.nih.