As was mentioned free range can mean different things to different people. To some people it means no fences of any kind, they are free to go wherever they wish. To some it means they can get some forage but are much more limited. Legally, when you see it on a package of chicken meat or eggs it just means they can go outside to a tiny bare patch if they wish. It doesn’t even have to have any vegetation in it. I think for most people it generally means they can at least forage for some of what they eat.
Different people perceive different benefits to free ranging, depending some on definition. To me if the quality of forage is good it can help reduce your feed costs. It’s a more natural way to keep them and with all that room you hardly ever have behavioral problems. The more room they have, whether total “no fences” or a really large run, the less likely you are to have parasites or diseases since they are not as crowded. I’m using the definition that they may have fences but there is a lot of area in those fences.
The disadvantages are that they are more vulnerable to predators and they can go where you don’t want them to. That might be into a road, in your garden or maybe they scratch in your landscaping beds, or they poop where you don’t want them to.
About the only way to determine whether they wipe out all vegetation or not is to try it. The square feet per bird you need to keep vegetation will depend on your climate, time of year, soils type, kinds of plants, and such. Whether you are on a north facing or south facing slope can have an effect. Age of the chickens can make a difference, chicks don’t need as much forage as adults. We are all unique in these. You will get different results if you are in the Sonoran Desert, Minnesota, or the Gulf Coast.
You can go through three different general conditions depending in all these things. For many of us the run becomes a barren dust bowl. The chickens eat practically everything green and scratch up the roots to eat those too. It may take them weeks or months to get to this stage but it’s where a lot of people on this forum are. I don’t consider this free ranging.
The next step is where you keep some green stuff. The chickens have a preference for what they eat, they much prefer some plants over others. So they selectively eat what they like and ignore what they do not. It’s kind of where I’m at, I have a large area in electric netting but don’t allow them to go wherever they wish. The stuff they don’t like grows up and smothers out the stuff they do like since they keep the good stuff eaten down. So I have to go in there a few times a year to mow it back to allow the good stuff to grow so they have some forage they like. In the spring I lock them out of that area for a week to ten days when the grass starts growing to let it get a head start. There are some bare spots fairly close to the coop where they like to dust bathe, but in season it is usually pretty green. It helps when I get rain.
Then the ultimate is where it’s just a pasture. They have a lot of room. You will still get some bare spots near the coop or where they like to hang out and dust bathe, usually in the shade, but the whole area continues to provide good forage. How much you need to mow, if you do, depends on you. Where you are I think it would turn into a jungle if you don’t mow or bush hog it down occasionally.