That depends on the bird.
I had a Brahma who, after she hit POL, was so heavy that her maximum flight capability -- when powerfully motivated by the thought that someone else was getting a treat she wasn't sharing -- was about 18".
I have seen a 6-month-old OE cockerel make it up to a 10-foot roof by stages and his brother make the top of a standard-height door in one flight.
I find that the worst birds for flying out of the run are the POL pullets.
At least 4 feet, maybe 6 feet for lighter birds. But there are ways to make them less likely to go over any given fence. Primarily, to make the top too flimsy to land on because few birds (other than my first California White), intentionally go up and over. Usually they either go up because they're startled or something and randomly come down on one side or the other or they go up, perch on the top, and then decide which side to come down on.
(That CW, Chipotle, was smart enough to fly back into a pen she flew out of when she was done with whatever she was doing out there).