Gday
This is a much debated topic. Many pilots just fly and keep an eye on the props. Others record flight times on them. There are many factors that can deminish the life. I found i got 25 hrs flying out of mine before i observed a slight crack forming in the plastic near the rivets on one prop only. I chose to change them all cheaper option than loosing my drone. I use a jewelers eye glass to examine the props for cracks after im done flying takes only a few minutes without it i would have missed that crack.