You should be able to work it out by filling up when the light comes on and knowing the total capacity if your fuel tank and how many mpg you get.
A lot of cars stop displaying when the fuel gets low.
Mine stops at 30 miles left and changes to -- and the fuel light comes on. I've gone almost 30 miles after the light has come on too, and didn't run out, so it seems pretty accurate.
Light comes on between 45-55 miles (left on trip computer) and will generally go to 0, and 15 miles extra!