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Legally it is the same as fixed wing; 1000 foot over congested, and 500 over sparsely populated. However, over areas where hogs would be hunted, the FAA never cares due to it being unpopulated.

Actually, for fixed wing aircraft, over sparsely populated areas or open water, you can fly as low as you want, but have to stay 500 away from any person, vehicle or structure. The law also says you have to maintain enough altitude to land safely in case of an engine failure, but, level flight over a flat field, that's only a few feet.