There are several factors in how much damage they will do. One is chicken density, how many chickens and how big of an area. One is the quality of the thatch, how well are the roots established and what plants are growing there. The time of year and your climate can have a big effect. How much rain do you get.
In Arkansas my flock consisted of around 7 to 9 adults in winter but in the peak of summer I might have 50 chickens (mostly young birds growing to butcher size) grazing in an area roughly 45' x 65' inside electric netting. I also had an enclosed covered run about 12' x 32' that was dust, no green at all. The area in the electric netting was never dust but in the winter it was brown or approaching bare. For the most part they never destroyed the roots like they did in that 12x32 area. I would water in the summer when it got dry. I had a few fruit and nut trees in that electric netting area.
They did quite a bit of their dust bathing in that covered run but some in that electric netting area, especially under a couple of certain trees. In summer they liked to lay in the shade of some of those trees so they kept the green stuff striped down pretty well. Grass and weeds generally don't grow that well in shade anyway. There were some bare spots, you would not like that in your lawn, but they never really stripped all of it. An area not under trees near the coop was stripped pretty bare.
When they foraged they would eat what they liked so that stuff would stay fairly short. The stuff they did not like would grow so I'd have to mow it three or four times a year to keep it down enough so it did not shade out the good stuff. In the spring when the grass just started to grow I'd keep them in that 12x32 area for about a week and a half to let the grass get a start. They really liked to devour those first tender shoots.
I have no idea how big a medium lawn is. The Kentucky bluegrass region in a wet spring/summer could carry a lot more chickens than an area the same size in the Sonoran Desert of the American southwest. Where you are matters. In many locations I would not expect 5 chickens to do that much damage to a decent sized well established lawn but I would expect then to have some bare areas for dust bathing if nothing else. I would not expect them to keep it down to where you don't have to mow.