Avocados have a notoriously small window of perfect ripeness. To see if we could broaden this time frame, we bought a case of unripe avocados and ripened them at room temperature and in the refrigerator three ways: on the counter (or refrigerator shelf), enclosed in a paper bag, and enclosed in a paper bag with pieces of green apple (fruit gives off ethylene gas, which helps many fruits and vegetables ripen more quickly). We also tried two more esoteric techniques: burying the avocados at room temperature in flour and in rice. In the end, the only thing that mattered was the temperature at which the avocados were stored.
At room temperature, rock-hard avocados ripened within two days, but many of them ended up ripening unevenly, developing soft spots and air pockets on one side just as the other side was ripening. After completely ripening, they lasted two days on average if kept at room temperature (stored in the fridge after ripening, they lasted five days). Avocados ripened in the refrigerator, whether in a bag or out in the open, took around four days to soften, but did so evenly. Stored in the fridge, they lasted a full five days before starting to show signs of over-ripening.
The bottom line: If you need your avocados to ripen sooner rather than later, keep them on the counter. Otherwise, for better quality, you’re better off putting them in the fridge and allowing them to ripen slowly. In either case, store the ripened fruit in the fridge to extend shelf life.