I agree with Steve -- there should be around 2.5 - 3 gallons left when the light comes on, based on my experience with other vehicles. So figure that you've got about 40 miles worst-case scenario when the light first comes on.
At the lowest level I ever let my Envoy get to the Driver Info Ctr computer showed a range of 59 miles, but I can't remember if the low fuel light was on or not. I think it was.
FYI - It's not a good idea to let the tank get below 1/4, and not only for the obvious reason of running out and being stranded. :lol: Below 1/4 tank the gas can slosh around enough that the fuel pickup tube is exposed (no longer covered in gas) which could cause stalling. Also, gas helps cool the fuel pump too, and more heat = shorter pump life.