Harvest cucumbers based on use. Harvest pickling cucumbers when they reach 1 to 6 inches long, slicing cucumbers at 6 to 10 inches long and cucumber for regular dills at 3 to 4 inches long.1  Dig This.t is ready for harvest in 50 to 70 days from planting, depending on how you plan to use them. Before cucumbers can produce fruit, the plants grow both male and female flowers on the same plant.