How long does a skin fungus rash usually last?

A long time. The most common problem with a fungal infection is that people undertreat it. Even after the rash appears to have gone, you should continue to apply your medication for about two more weeks. Fungi can live very deep in the six and you want to treat the rash ideally for one complete skin cycle, which is six weeks.
Years. Untreated a fungus can persist for months or even years and continue to spread from the original areas to other parts of the body.