PAX2908
I assume, in case of a positive result, it has to be fixed right away before returning it to service. Perhaps also some more extensive inspection is triggered? So in fact it would not (should not) be possible to learn how fast the cracks develop, from these inspections alone.
In general (not necessarily this one) That is what they do. These material have lots of tolerance to damage if inspected often enough. Some damage is always assumed (escapes from manufacturing inspections) and even with those you ought to make to even the longest overhaul interval one can imagine. However if the damage occurs in service (lots of reasons) than one needs some updated assessments of where and how much to redo it's life expectancy between specialized inspections.
In the end the expectations as approved by the regulator may not be 100% but they historically have done a very good job at preventing loss of aircraft when taken into context with the ruggedness of the aircraft to an engine failure.
So standby by for updated actions in this regard and hopefully some other aircraft type or engine model will not repeat these lessons learned.