How long does MRSA usually last?

90 days or 3 months. It depends on the host's immune system, interaction with other bacteria and environmental circumstances, exposure to antibiotics but generally MRSA can live up to 90 days or 3 months. A host can be colonized with MRSA indefinitely and not even be aware, therefore being a constant source of contamination.
Until treated. Mrsa is a serious infection that usually does not clear up on its own. It requires aggressive antibiotic treatment.