How long do food allergy rashes last?

It depends. Food allergy can produce immediate hive rashes that resolve within 1-2 days of eliminating the food. Other food allergy rashes, such as eczema can last for several days to weeks after elimination. Rash can also persist if there are continued exposures to the food in question. Finally you need to as, "are you sure the rash is due to a food allergy" see an allergist for evaluation.
Could last a life. Time. It depends on the type of food allergy and treatment given. Some allergies may be controlled with medication and tolerance may be induced to others. See this site for more info.