Somewhere between 10 and 35% of OG for beer. Depending on the yeast, the recipe, the mash temperature, decoctions, etc.
It's finished if it has been at 1.020 for 3-4 days.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
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