Till you run out of gas... 
Seriously though, seeing how each gas gauge is not calibrated quite the same the results would vary.
I wouldn't recommend waiting until the light comes on to fill up very often as you'll be picking up a higher concentration of "junk" out of the tank.
Seriously though, seeing how each gas gauge is not calibrated quite the same the results would vary.
I wouldn't recommend waiting until the light comes on to fill up very often as you'll be picking up a higher concentration of "junk" out of the tank.
