You'll find a dozen articles quoting "30 to 50 miles". The honest answer is that it depends on the car, and the only figure worth trusting is one you've worked out for yours.
Three ways to find out
Check the manual. Some state how much fuel is left when the warning comes on. Multiply by your MPG.
Watch the distance-to-empty readout, but don't bet on it. It's an estimate based on your recent driving, and it can drop fast in traffic or on a long climb.
Measure it once. Fill up right after the light comes on and compare the gallons pumped with your rated tank size. The difference is roughly your reserve.
Why it's not a habit worth having
Most cars have the fuel pump inside the tank, where the gasoline around it helps keep it cool. Mechanics have long advised against routinely running very low, and running dry altogether can leave you stuck somewhere unpleasant. The light is a reminder to stop soon, not a bonus range.
