I'm a big guy with no windshield. With the bike stripped down and no headwind, I can get 120+ with my 750 at 60-75mph before hitting reserve. Loaded and with no wind, 110. Loaded, out of tune and with wind, I've been as low as 90 miles when it started sputtering.
The best way to know, fill the tank with gas, ride till you hit reserve. Fill it up again and record the mileage & fuel used. Ride until you hit reserve. Fill it up again and record the mileage. Do this 20 more times and you'll have a pretty good idea how widely the mileage can vary.
From there - check early, check often. I like to fill up before 100 miles and seldom push it to 120, if I don't have to.
my VT1100 will normally hit reserve at 120-127.... but I only run it that low if I am around stations as its only got another 12-14 miles till empty
This weekend been I did some group riding and it was slow and I could have gone 140ish before hitting reserve.... I filled expecting to need to before the next fuel stop and I had only used half a tank
My 2002 VLX gets 52.5 MPG. I kept track of every fill up over the course of 3 months. So with a 2.9 gallon tank I could go about 152 miles total. I ran out once....I'll never do that again. I look for a gas station about every 100 miles.
Like Jwinker stated, lots of variables, for instance, my vlx has a 41 tooth sprocket,,I have a plexifairing 3 windshield(big), I weigh about 142(finally gaining back lost weight). Before the windshield I have gotten as many as 119 miles on the odo before switching to reserve.
The biggest variable of all "your" right hand,,ie how you come off the line, how far you wind out each gear. You can ride for the thrill and get crappy mileage, or high speed for extended miles will cut into your mileage. On one of my need to get there fast rides of 150 miles at 85+mph, I only got 37 mpg. So you see, it
all depends on how you ride, your bikes set up etc...