-15ft. I don't let it go flat. I'll bail off the bike as i come to a stop before taking a chance on the rim cutting the tire, or the rim getting a scratched up.
I've had a slow leak on a pizza/bar hop ride and let it go a couple miles. Front tire, so I rode no handed leaning back. It never went flat, just very bouncing with no weight on it. Had to stop once to throw some air in it quickly, then changed it at the next stop. I didn't know where the place was, didn't want ot ask the group to stop, and wasn't ready to head home yet.