On a charge, 15 miles but depends on your weight and the terrain. My son put 22 miles on one once on a single charge.
Safety wise, yeah wrecks on onewheels can be bad. If you are a speed demon you will break something. If you are careful you should be fine. A set of fangs on the board and a pair of wrist guards are highly recommended. And a helmet at all times.
Onewheel XR. Thought they were dumb when I first saw them. But they are super fun and anytime I can go out and ride it makes me smile. I have ridden 300 miles already. In snow, dirt, sand and streets.