This info is for U.S. model R3s. It could be different for other countries............
When the last bar on the fuel gauge starts flashing you have about .79 Gallons (3 liters) left in the tank.
The gauge then automatically changes to "Fuel Tripmeter" mode and begins from zero to count up the distance traveled from that point. You should then safely have about 30-40 miles (48-64km) to go until the tank is dry. NOTE: this depends on: how fast or aggressively you are riding, headwinds, etc.