The Aprilia RST 1000 Futura has a stated tank capacity of 21 litres and is designed to run on 95 RON unleaded petrol.
Reserve kicks in at less than four litres, when the light comes on.
Fuel consumption varies from bike to bike, and also how you ride. Expect around 140-160 miles to reserve from a sidestand fill up and steady riding, but be aware that the bike gets much more thirsty with prolonged high speed running, and drinks petrol at 100-110 mph and above.
These Rotax Vee twins are known for drinking petrol if you thrash them, getting really thirsty at 6,000 rpm or above.
It’s a recurring topic of conversation on the Apriliaforum, so you can compare notes with riders all over the world on this thread: