How long does it take melanoma to spread?

Varies. Each cancer is unique. Some cancer cells have the ability to metastasize very early in their development while other can be slower to metastasize.
Depends. It depends on the depth of the melanoma, and where the melanoma is located. Generally, the thicker the melanoma, the higher the risk and velocity of metastasis. It can vary from decades to months, so it is very difficult to predict.
Variable. Course of tumors is very difficult to predict. Some tumors are already widely disseminated when first diagnosed.