Depends on the size of the gas tank and when it trips. One car I had trips at 1/8 tank, another at 1/4 tank. I would not count on more than 30-50 miles. If you run a fuel pump with the tank low for very long, the pump can run hotter and it will shorten its life. It depends on the fuel around it to keep it cool. That's why I don't let my cars get under 1/4 tank, and I usually get 200,000 or more miles from each pump.
It all depends on the car. Since this is the PT forum I'll guess you have a PT, is it a turbo, GT, regular, auto or manual? Each one will get different millage and it will also depend on how heavy your right foot is. Our 06 base 5 speed regularly averages 29 mpg with a 15 gal tank and when the light comes on there is about 3 gal left. A good rule of thumb is to never let the tank get empty and run the pump dry, any sediment and or water that may be in the tank will get picked up and wreak havoc.