The best way to quickly ripen an avocado is to place the avocado in a brown paper bag with a banana or apple and leave it on a counter or windowsill for 24 to 48 hours. Avocados that are hard or firm may take an additional day to ripen.
Avocados produce natural ethylene gas, and placing an avocado in a paper bag contains the ethylene and helps the avocado ripen faster. An avocado is ripe when its skin turns dark green and when it is slightly soft and yields to gentle squeezing. Avocados remain ripe for two to three days.