My light usually comes on after I've used around 14 litres of an [alleged] 19 litre tank, so I figure another 1/3 on top of what distance I've already done. I reset my trip meter to zero at every fill-up, so I can see how far I have travelled, which gives me an indication of gas mileage for that tank. If I've done 240km [which would be about normal for mixed use], then I've got another 80km left. Or in the Northern Hemisphere, 150mi travelled would leave me another 50mi in the tank.
I haven't tested this hypothesis to its very limit - warning lights make me nervous, so I look for fuel pretty quickly after the light comes on, but I know I've got a bit of a buffer there, if things don't go my way.
[J/k about the hemisphere; I know km and miles are all over the place]