I haven't really thought it through but off the top of my head 250m from a house just passing by seems reasonable, and maybe something like 750-1000m for biouvacking? No one (who wants the right to roam in the wilderness) desires to roam around the suburbs, and having some reasonable limits like 250m/1000m would prevent people from doing that. It would prevent people from entering tracts of land that are even a few acres in size. When I think "the right to roam" I am thinking about places with vast tracts of wilderness. Think some "ranches" in the American West, though could be tens of thousands of acres of land not really used for much besides grazing cattle. No one advocating for the right to roam wants to camp in your backyard.