The Honda CBR250RR has a 13 litre (3.43 gallons) fuel capacity. The reserve tank has a 1.6 litre (0.4 gallons) capacity, which sees a maximum mileage on average of between 180-220 kilometers (111 - 136 miles) before switching to the reserve using the Fuel Tap. It is advisable to go to a petrol as soon as possible once the main tank is depleted. The reserve has a range of about 25km (15 miles).
When sitting on the CBR, the fuel switch is to the left around the position of the left knee, just under the seat. It is a simple tap with three settings - main, off and reserve. Be advised that because off is in the middle, switching to reserve means clicking the tap around twice. The switch is easily reachable when riding.
The Honda 250RR being 10-18 years old is not tuned for premium fuel. Regular unleaded is perfectly acceptable and no performance or otherwise gains will be made by running on premium.