As a propulsion technician of over 40 years maintaining and overhauling gas turbine engines, I am used to boroscoping various parts of the engine. In my experience, some defects/cracks propagate quite slowly and can be monitored safely... i.e. combustion chamber liners and HPNGV vanes. Other defects, particularly on rotating components fail within a very short time (a few hours) of the defect becoming visible to the naked eye.
I would have thought that a crack on an LP fan disk would go from "visible" to "failure" very quickly. Perhaps someone better qualified in that field could comment on that? I'm guessing that the mandated inspection of the engines must be NDT rather than boroscope or visual to be of any value?