I often ride up to 45 miles after the low fuel warning comes on, and normally fill with 18 or 19 litres, but don't rely on other's as a guide to what yours will do, fuel gauges are notoriously inaccurate as do riding styles/fuel consumption. It's best to judge it for yourself, topped up to the bottom of the neck in the tank there will be 18 litres available... if you only get 13 in that means you can delay filling for 5 litres-worth.
