How long is a piece of string? It's entirely dependent on speed, rpm, inclination, wind direction, how much of a load you're carrying, the alignment of the stars etc etc etc. As a rough example, when I'm on a motorway at 70 and the light comes on (and stays on) I've managed to go another 30 miles at that speed. Of course, if I slowed down to 50 and stayed in 5th I could probably have gotten a lot further ...