If someone had an untreated melanoma, how long would it likely take before it starts to spread to other parts of the body?

Variable, unpredicta: Melanoma can be unpredictable and is quite variable in its behavior. If you can tell us the exact information about this melanoma such as a biopsy report and or the stage of melanoma, then we can be more exact in discussing its prognosis. But it is all dependent on the stage of the disease with some variability among different people.