I deliberately ran mine out a couple of times to find out how far I'd get after the light comes on.
Just a bit over 40 miles in my case - I'm sure individual results will vary widely.
Carry a small gas can and find out what yours does.
Just a warning though - when mine ran out of gas, it was like turning off a light.
One second it's running, the next it's stopped - didn't get any coughs or splutters, it just stopped.