The fastest way to ripen an avocado is to place the fruit in a brown paper bag with an apple or banana and set it in a sunny area. The warmth of the sun and the ethylene gas produced by the apple or banana help accelerate the ripening process.
Avocados do not ripen on the tree; they ripen after they have been harvested. If an unripe avocado is not needed right away, place the fruit on the counter for a few days. A very hard avocado takes three to four days to ripen if left out of the refrigerator. When an avocado is ripe, it gives slightly to pressure. Once it is ripe, it should be used within two to three days.