Driving down the highway at about 70mph, I kept an eye on it. I went 65 miles from when the light came on! I had gas with me if I did run out, but after 65 miles I figured that was enough. Good to know!!
I have heard people say that the level should be about eight gallons remaining when the light comes on. I know I've pushed mine to the limit a few times driving across New Mexico, trying to avoid the 86 octane garbage that they sell there.
Scangauge will tell you exactly how many miles left and how many gallons. I have gotten mine down to 2 gallons left, but that is as far as I am willing to push it. No telling how much unusable fuel is in there.