Usually about 21 - 30 days after planting. If you take your forefinger and thumb and probe down below the soil level around the stem of the radish, you should feel a bulb. When the bulb is about the size of a store bought radish, harvest. They need a lot of water to make those bulbs and don't like hot weather so if they start to grow stalks, they are "bolting" and this means they are going to flower, to make seeds. These are tough and old radishes. Usually where there is a stalk, they have no balls.