Generally, watermelons reach harvest 65 to 90 days after sowing. As might be expected, smaller varieties will usually be ready before larger ones, but not always. Some early icebox-size watermelons, of 2 to 4 pounds, Early Midget, Garden Baby, Golden Midget and Little Baby Flower, mature in 65 to 70 days.lanting Watermelons. The ideal date to plant watermelons varies according to the climate of the region. To produce the sugar necessary for sweet fruit, the melons need hot weather. Seedlings should not be transplanted until three to four weeks after the last average frost date in the spring.