An avocado is eaten when it is ripe: giving under the thumb, neither hard nor soft. Once open, it browns fast, a little lemon protects it.
Giving under light pressure, with no soft dent. Remove the small stem: green underneath, it is good; brown, it is overripe.
Cut it in half lengthwise going around the stone, twist the two halves in opposite directions to separate them. Remove the stone with a spoon, not with the knife blade stuck into it. Scoop the flesh out with a spoon, or cut it into dice in the skin before scooping.
One minute.
Hard, at room temperature, three to five days, faster next to a banana. Ripe, in the fridge, two or three days. Cut, with the stone, lemon and film against the flesh, one day.
An avocado mashed with a fork on toast, with lemon and salt.
Never stick the knife blade into the stone to remove it: when it slips, it ends up in your hand. A spoon does the job.