This is going to depend on where and how much you fly. I have two drones and fly about 10 hours a week time split 70/30 between the two drones. I inspect my props before every flight inspecting the leading edge of the props and making sure there is no play where they mount to the motors. It doesn't take much for props to become chipped and unbalanced. I also put them on the balancer after every few flights, unbalanced props cause wobble which can add fatigue to the arms of the drone. This is a good question and my answer is it's inspect them often don't wait till they fail in mid flight and use good judgment. Under no condition would I use a prop if it has struck the ground or any object.