Is it unusual for a rash due to a food allergy last for several weeks?

No. Some skin reactions due to food allergy can last for weeks, especially if you are still being exposed to the food. However other things can cause long lasting rashes. I recommend being evaluated by an allergist or dermatologist.
No. It may last years. See this site for info. http://www.ncbi.nlm.nih.gov/pubmedhealth/pmh0001820/.