On most cars it's usually keyed to come on with 5L remaining, having said that it will often flicker or randomly trigger at around 8L, depends on if your driving level surfaces etc.
It's supposed to indicate the 'emergency reserve', so a solid on means you should be looking for a gas station, in practice for small cars you can push it a little, I know I've had them come on in the middle of nowhere miles from a station.
But, just a general warning, submerged in-tank pumps rely on that for cooling, to be submerged, and when your that low you also risk the pump sucking air in a hard corner/stop etc, which is actually 'really' bad for a fuel pump.
So I know I play it safe, my fuel light comes on, I get fuel.
Edit: To follow up what Frank said, I believe the warning light usually uses an extra sensor, something like an air sensor sitting very low in the system, separate from the float, so yeah leaving it out likely saved a moderate few pence.
Second Edit: My base model 95 LX has one, with driving on hills a lot I trust it more than the gauge, which I find is overly dampened, so sluggish to update it's actual level after a trip up or down the mt roads.
