The approximate answer for the cars that I have driven that take the standard 5-6 quarts is "about 2 quarts low". Maybe more, maybe less. Normally, you'll notice the light come on when taking a sharp corner as the oil sloshes to the side and the oil pump sucks air for a second.
But, the major point here is that the light means "low oil pressure", not just low level. That's important because low pressure means oil is not pumping through the engine properly, and serious damage isn't far behind after that. When the oil is too low to cover the pick-up tube from the oil pump, it sucks air, resulting in low pressure and "less than optimal" engine lubrication (to say the least).
If you see the light come on when taking a corner, find the nearest place to buy oil and add it. If you see the light come on when driving normally, pull over and turn off the engine as soon as safely possible. Don't drive the car until oil is added.