I think the car is usually a quart low when the light comes on. There is a good amount left, but you really shouldnt drive too long when it's on obviously. They wouldnt design a warning light that only comes on at the ablsolute limit of it damaging things.
Mine flickers when it gets down to about 2 quarts. I've only done that once. My celica the light came on when I had about 3/4 of a quart . My wife tells me this after she's been driving it like this for 2-3 days :sadance: .