not sure when the light comes on, but i've had the needle a little below halfway between E and the 1/4 tank mark and no light. when i went to fill her up, 13 gallons went in. our tank holds a little under 17 gallons (like 16.9 or something, whatever 64 liters is equal to). but yeah, u should always keep it at least 1/4 tank.
My low fuel light comes on exactly when I have 1.5 gallons left, because, when I fill up at that point in time, the pump allows 16.5 gallons. On a couple occasions, I drove several miles with the light on, but the most I ever needed to fill up was 17.2 gallons. I was cutting it a little too close there and I definitely don't want to damage anything, so I'll cut back on being so stingy with fuel. I like to get at least 400 miles out of each tank.