I have just over 6,100 miles now on my 2007 Nightster. I bought the bike in April of 2007. For the first few months, I ran it stock and the gas light would come on at around 80-miles just like clockwork. I would fill the tank, and it would take about 2.2 gallons. I could average around 100 miles per tank of fuel.
After a few months, I changed the pipes to Vance & Hines, the intake to the Scream'in Eagle, and had the computer chip performance upgrade done at the dealer. Since then, my fuel light comes on at around 85 miles, and I can get a maximum of 110 miles to a tank.
The mileage isn't much of a difference, but before the upgrades, the bike topped out at between 110-115Mph. Now the bike can bury the odometer needle past the 120Mph mark.
Bottom line, if someone is getting 100 miles before the fuel light, you must be running the 4 gallon tank, because on the XL1200N 3.3 Gallon peanut style tanks, I haven't really seen anyone get past the 90 mile mark without the fuel light coming on. My rule of thumb is that I just reset the odometer at every fuel stop, and when I'm hitting 90-miles, I fill up again.
Hope this helps.