Optimistically you should have two gallons (8 to 10 litres) left when the light comes on. Problem is, that varries since the light is triggered by the tank sensor dropping below a certain point in the tank. Sometimes, that level is a bit lower and you may have as little as gallon left when the light comes on.

The best way to find out, without walking, is to fill the tank and drive until the light comes on and refill. Then look at how big the manufacturer says the tank is, find the difference and that is how much fuel you should have "in reserve". As a just in case, I would take a small can of fuel with me, because there may be an issue such as a deformed tank that reduces its standard volume.

In many ways how much is in reserve doens't mean much. You need to know how well your car does on fuel consumption, your speed etc before the information becomes useful. Knowing that you can travel 800 miles on a tank of fuel at a certain speed is only important if the stations are 790 miles apart.

My dad used to tell me that a car traveled much better on the top half of the tank than the bottom. In those days, stations were often miles away from the farm in our part of the rural Canada and what he meant was that he felt better knowing he always had enough fuel to reach the next station regardless of what happened.

I had one car that had a reserve light and no matter how long I went without filling the light never came on. The day I found out that the sensor in the tank was faulty was the day I walked a couple of kilometers to the gas station. The level indicator worked fine, just the low fuel light would never come on. I now follow my father's sage bit of wisdom faithfully.