My chickens made the decision when they were ready for true free-ranging. Up until about 15 weeks old they were quite content to stay in the yard, close to their "comfort areas"; a large bush and a corner of the house where they would run when frightened. One day recently they decided to check out the world on the outside of the fence and they haven't looked back. Yesterday when I went out to check on them I found them really far off. I'm no good at describing distances, but try to imagine our house sits on a 1/2 acre fenced area, the fenced garden space next to that is a little over a 1/2 acre, and they were in the meadow on the other side of that! They start working their way back towards the house and their pen at dusk. The only thing I've really had to worry about is our cat. She's very small, only 2 and a half pounds, but she's already proven she can't be trusted around the chickens. She's become an indoor cat while they roam, then I put her out at night after the chickens have gone in to roost.