Not really a set time to replace no. But,,, make sure you check them before every flight for cracks or knicks in them. Even a small knick will cause it to be unbalanced leading to premature wear on motor bearings. You videos will suffer too with vibrations.Newb question on props....other than a faulty or broken prop...when do you replace a prop? Is there a number of hours of use before they should be swapped out?