My flock free ranges, and has access to over 100 acres. However, they have never strayed more than 3 acres and generally stay within about 1-2 acres around the coop. When I first start letting chicks free range, at around 4-8 weeks, depending on time of year & weather conditions, they stay very close to the coop for several weeks and only start venturing further than 50-75 feet from the coop much later (~3-4 months old). 
I agree with the need to watch out for fences. I've had chickens "break into" temporary fencing around the brooder coop, temporary fencing put up for breeding purposes, and the chain link fence we have around a cell tower on our property (no idea how that silly rooster got over it to get stuck inside). Fences are very confusing for them, for some reason, so you have to keep an eye out. I even came out one day to find a Welsummer hen upside down, immobile, with her leg caught in a temporary fence (she was on the outside of the fence). The only thing I could figure was that she was fending off an advance from a young cockerel and got tangled? (She ended up with a sprained ankle that healed on its own in a few days, while she kept herself in the coop).