This depends on the kind of plants you would grow, as some require a thicker layer than others. Assuming a layer thickness of 10 inches (good for e.g. spinach, tomatoes, sukumawiki, broccoli, etc), and a density of soil of 2000 kg/m3, the area would be 1.6 sq ft (0,15m2). We advise to use the soil mainly for vertical farming technologies, houseplants, flowerbeds, etc. and not for large surfaces.