The maximum distance is the amount of pipe you have. You should be concerned with the minimum. That is established by your building or fire department. It will be so many feet from the building or any open flame, which is what your heater is. Probably at least 20 feet.
Another thing to consider is LPG is heavier than air; this will make it fill depressions in the building like basement areas if there is a leak. Ensure joints are sound.