Welcome to CRF's Only and congrats on your new ride! You are correct the fuel capacity is 1.66 gallons. The ride time depends on how you ride it of course so your mileage WILL vary. Best to take it out with a gallon in the tank and run it dry then calculate for 1.66. You'll hear anywhere from 35 to 60 per tank. A good average is likely 25 mpg. Again, all depends on how hard you riding it.
Again, welcome.