I find my light will pop on, and ill do a couple of miles, then next time i start car it wont be on. Once its on for good ive done 20-30 miles without running out.
All depends on yr driving style, when im low...and cant afford to fuel up i drive more economically than usual.
Long journeys i can do 430 miles on a full tank...around 10 gallons....do the math
Driving until the light comes on should be avoided though. It allows more of the gunk from the bottom of the tank to foul up the filter (or your injectors if you are unlucky)