I just searched on this yesterday and there are a few... general consensus is that 40mpg seems to be the norm. So about 120 miles before reserve...
If you're getting less, then you ought to look at things like the brakes ~ are they dragging? Chain and sprockets ~ adjusted correctly, lubed and not excessviyl worn? Wheel bearings ~ lubed and spinning freely? Then you also get into the general tune of the engine.. are valves adjusted, carbs not running overly rich, air filter properly oiled and not too dirty, petcock and fuel needles in good shape so gas isn't flowing when the bikes off, etc...
Then there's Always the last thing, riding style... do you use a lot of gas and brake? Or are you smooth on the throttle, coasting to stops and rarely in higher RPMS. I know I can alter my cars MPG by almost 8 mpg in Just driving style (old man mode vs. punk kid mode).