How long do I need to wait after winter reseeding my grass til I can let my chickens on it?

Does anyone have experience? Here in Phoenix we can plant winter grass. We just put seed down yesterday. We only have 3 hens and I only let them out of the run after school until they go back in at dusk. I just dont want my grass destroyed, but at the same time, you can tell they want out!

 

Thats what I thought... poor birds.... (They have thier run, and coop so its not like a chicken factory, but stilll....)