How long does it take for a mole to become cancerous?

Hard to say: There are many kinds of pigmented lesions that are called moles. Most are absolutely benign, so i can't give a time frame. There are estimates of how long it takes different types of early melanomas to become invasive, but that's a long answer. The best thing to do is to see your doctor and have a skin check. Tell them about any moles that worry you, so that you can be relieved.

May be from start: The vast majority of common moles present no cancer risk. Any one that stands out needs to be evaluated. A new one that looks different from its neighbors may be malignant from the beginning.