Not Much. Get to a gas station. I would think the amount verys slightly from when first come on the only way to say for sure would be to run it dry add gas slowly until it goes off. My bet is its about 2 gallons.
Well I have a 30 gal tank on my 97. When the light comes on I normally get to a station. When I fiil the tank it always takes 26-27 gallons to fill. So if you subtract the amount it takes to refill the tank from the tank capacity it will give you a pretty good idea how many gallons you have left when the light comes on. Mine is around three gallons.
I once ran my '98 until it started sputtering. The fillup was 25.2 gallons, on a rated 25 gallon tank. I got 45-50 miles after the light, which would put it right about 3 gallons. In my opinion, it comes on too early, but it has probably saved a lot of people that don't pay attention to their gauge.
the light on my 05 xlt always came on right as the needle touch the red empty mark... my 06 lariat comes on right when the message center says 50 miles left, however some times the needle is touching red and some times i still have 1/8th of a tank when it comes on.