I've heard you have about 40 miles until the car runs out of gas, that is if you get the averaged MPG. I believe the light comes on with about a gallon and a half left, so if it's a 15.4 gallon tank it might come on at 14 gallons.
Basically if it comes on make sure your next stop is to a gas station. I have had the needle touch red, and a little after the light come on, however I live right off of what they call gasoline alley due to the numerous gas stations around here.
Normally I'd try to fill up at 1/4 of a tank, but I've been trying to hold off on filling up as often.
And you might like this: http://www.motorcraftservice.com/pub.../02musog3e.pdf