That my brother is gunna be up to how much you put in just after the light comes on. There is no rhyme or reason to it!
My low fuel light comes on after about 150 miles which is not too bad. It takes about 4 gallons to top it up which means low fuel light indicates 2.6 gallons left. I'd prefer it came on with a gallon and a half left.
Mine comes on at about 120 miles... almost like clock work. I'm guessing about two or two and a half gallons left. I've never ran the bike dry but it should get 180 to 200 miles on a tank... depending on how much throttle you use....
Managed 206 miles between fuel stops last summer in Sweden at speeds of between 60 and 80 mph, however the roads in Sweden are long and flat so the throttle was only open very slightly.