The simple correct answer is, "I don't know." But the bullshit internet answer is that there is no real risk of metal fatigue, all you have to worry about is permanent deformation of the bar, or in simple terms bending so far it doesn't go back to center, maybe just plain breaking. I don't think many have reached that point but that's the bullshit internet answer.
Two different things going on there...fatigue would be failure due to repeated cycles. That is unlikely, as its a "spring" of sorts and designed to be twisted, over and over. You could plastically deform the thing, which is twisting it in one direction further than the elastic limits of the steel. Also unlikely, in my mind, as the forces you would need to do that are much greater than that which the jeep can generate...that's an educated guess though. I haven't done the math. Most likely, the limits to travel would just show up as a limit to travel...no harm, no foul. at a certain point, the jeep would simply not be able to exert enough force on the bar to make it twist any more. Its not heavy enough.
It is a spring essentially no different than a clevite bushing with similar life span attributes in that the number of cycles it can endure before failure is directly related to the frequency and amount of deflection. The main thing keeping the AR alive is the arm length. The longer arms reduce the degrees of rotation which keeps the spring in elastic deformation.
