Low fuel light

I was wondering how many gallons of gas do you have left in your tank when low fuel light comes on. I have a 2000 gmc sierra reg cab long box. I was wondering because I'm trying to figure out my gas mileage. I never fill the tank up because its a 34 gallon tank and we all know that would cost over $100 dollars to fill. Thanks to everyone, you guys are always a big help.

It varies model to model, 3 gallons seems to be the norm.