It should be said that at that point you don’t have very many charge cycles left after the capacity drops below 80%, and the capacity will drop a lot faster for every charge cycle after that point.

I don’t have exact numbers.. based on graphs I’ve seen I would guess that if the original cycle life was 1000 cycles you may have another 500 cycles until the battery is actually unusable. But it probably depends a lot on the specific chemistry and how the car is used.