I buy them rock hard and then eat them as they ever so slightly ripen (I like them very firm). A bag of small ones last me about 5 days and I keep them on the counter. SusanW's answer is great if you are not going to eat them quickly.
If I get a good deal on them, I put some in the fridge which nearly stops the ripening process. I also put cut avocados in the fridge. If you cover the exposed flesh with water, it completely stops the browning process and the water pours right off.
Either way works. You can put them in hard as rocks, partially ripe or fully ripe. My mom's property has 32 avocado trees in the backyard. She's been doing this for decades.
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