It depends on the car, but it typically comes on when you start dipping into your reserve tank, which is what varies... in my old E320, the reserve tank was roughly 1.7 gallons (from what I remember), whereas in my brother's Civic, the reserve tank is under a gallon... but keep in mind the differences in fuel consumption between the two cars. If you HAVE to, you could go 20+ miles from when the light first comes on (though sometimes it goes on and off for the first couple miles), but if you drive that far, I'd start seriously looking for a gas station. Don't make a habit of running on fumes, since it can be bad for your fuel system in the long run.