There's some pretty good info and questions in this thread, which brings up some fun points to ponder.
Regarding our plastic props; unfortunately I doubt any significant life cycle testing has been performed on them to establish a 75% or 80% life cycle period. When dealing with aviation components we always want to replace them prior to reaching the end of their life cycle. Failing to do that means the life cycle of the part is over when the part breaks, which in turn carries a high probability of that point also becoming the life cycle of the aircraft as well. When the component fails it takes the aircraft down with it.
What's 75% of life cycle? Since manufacturers do not publish life cycles, for reasons we can only surmise, each of use becomes a guinea pig. Because of that we should perhaps be performing more frequent inspections. Instead of inspecting props at the beginning and end of a flying day it might be better to inspect them before each new flight. Since environmental conditions vary widely, where and how we fly influences how often we should inspect our props. Do we fly in a very high insect count environment? Fly through swarms of bees or over decaying kelp beds with high fly counts? If so we'll experience some level of plastic erosion that has to be watched over. Do we land and take off in tall grass often, or get a bit sloppy with our landing techniques and run prop tips through tall grass? If so we should be looking for chips and splits at the tip of the blades and cracks at the prop hubs.
Since we don't have a published life cycle we get to determine when props should be replaced through a systematic inspection process. We'll check the security of the prop locks for free play. Those that leave the props on will see their prop locks last much, much longer than those that install and remove them every day. What might be considered a failing prop lock? If it makes you feel uncomfortable when doing a wiggle test it's definitely time to replace. If you can hold the motor still and cause the prop to separate from the motor with not too aggressive reversal of prop rotation it's time for a new one.
Next we'll look at the blades. Chipped tips? If small and we have a prop balancer and know how to use it we might be able to smooth out the chip with some #400 sand paper and re-balance the prop. However, that only has a chance if whatever caused the chip didn't also introduce a crack or split of the blade. We'll look for those by using a light opposing twisting motion with our fingertips on each side of each propeller tip. Did the prop appear to hit something pretty hard but not break? Time to check the tips of course but it's also time to check the blade hubs to see if any cracks have been introduced. A seriously out of balance prop or one that has experienced too many flex cycles will show itself at the blade hub with what is at first a small stress fracture where the blade meets the hub. Those usually spread somewhat radially until the blade separates. Once started they progress quite rapidly and always end catastrophically.
Those that are tracking every flight and recording the times will be the ones to provide the data to establish life cycles. Some of them will be commercial operators, some of them dedicated amateurs, buy what we want to know can only come from those that are dedicated in recording sorties and times. Unfortunately the H has not been out long enough to accumulate enough time on numerous individual aircraft to put time between failure numbers together yet. Props that are not abused can last a very long time so for now all we can do is perform good inspections to assure they have not been damaged by external objects or worn out from numerous removal and installation activities.