With 2 hens in a 4x7 (footprint) tractor, I have to move it daily or at MOST every 2 days in order to prevent serious conspicuous damage to the lawn. As it is, you see a trail of eaten/scratched/dug/pooed-on rectangles stretching out behind the tractor
This is on decent lawn, it is a bit weedy and not super lush but the soil is pretty decent and moist, it doesn't brown off too bad most summers. On my lawn, it takes 2-4 wks before the tractored spots become TOTALLY visually-indistinguishable from the rest of the lawn, depending on how well the grass is growing at that time of year.
So, I am going to guess you'll need to move your tractor daily, or at most every 2 days, depending on what effects you can tolerate.
Mind, even if they do completely devegetate a spot, the grass *will* grow back (unless they do it real real bad and your grass was in poor shape to begin with). But it may take some months, and the tilth of the soil will have suffered.
GOod luck, have fun, maybe consider a permanent coop-and-run
,
Pat