To ripen firm kiwis, leave them at room temperature, but away from heat or direct sunlight, for a few days to a week. Hasten ripening by placing them in a paper bag with an apple, pear, or banana. Once a kiwi fruit is ripe, however, store it far from other fruits, as it is very sensitive to the ethylene gas they emit, and tends to over ripen even in the refrigerator. Ripe kiwis should keep for about one to two weeks.