This really depends on the type of potato you’re growing and the size you want to grow them to. New potatoes need between 60-90 days to grow, whereas full-size potatoes require about 120 days. You can tell they’re ready to harvest because the leaves start to die back and you might also notice a small mound of soil pushed up at the base of the stem. Failing that, you can also feel for the size of the potato in the soil if you’re unsure.