Chicken Run over grass: move how often?

[MissFoodie]

Am considering chickens and although they could free range 3 days a week the other 4 they will be in a run. Plan is to have run over grass, then once ruined, move run to allow that piece of grass to recover.
Question is: how often would I have to move run? Eg how long before the grass becomes muddy pooey mess?
Assume chip bark would help?

bark chip would kill the grass and get spread everywhere, if you only have two or three and move it once a week it should be okay though.