This can vary. Avocado will brown when exposed to oxygen. To help preserve it, spritz an acidic ingredients such as lemon juice on it. If it turns slightly brown it is still edible. I just scrape off the top brown layer and eat the green fresh flesh underneath. However, use your best judgement....if it's been sitting for a while and is really mushy or the brown is extremely deep and dark, then buy a fresh avocado.