I would look in the owners manual and see how much fuel is there once the reserve is hit. Usually, its .9 gallons. Your bike should average around 35-40mpg, depending on how your riding. So, theoretically, right around 30 miles. But, realistically, anymore than 20 and your really going to be pushing it on any bike. There is always gas in the tank that doesn't get used, so that .9 amount is less than that. I know I ran out on my old F3, and it was right around 15 miles after the reserve. I can't remember what the reserve was on that bike, but I think it was .9.

Your best bet is to figure out how many miles you can get from the tank, then reset the trip meter everytime you fill up so you'll be better prepaired. If you pass a gas station at the 100 miles mark on the tripmeter, you'll know you should probably pull over and fill up instead of hitting the reserve with unknown miles until the next gas station.