It's best to set one of the trip odometers for fuel range, I have found my fuel light to be very accurate. I have about 1 gallon left when the fuel light comes on.
How accurate are these fuel gauges? Ran a few miles today with low fuel light on. Put in 3.5 gallons, and gauge read a little beyond full.
You already have the answer to your own question! They vary, my older bike has less than a gallon in reserve. You now know that you have approx 1.5 gallons in 'reserve', so also know how far you can push things before you come to a fuel-starved halt. When using my bikes locally or on known routes I will ride until I'm down to around 1/2 a gallon before refilling, however on unknown routes or when on that grand tour I start looking for fuel at around half full, before turning on reserve.