20 years. A complicated technology like present aircraft, nuclear energy, petrochemical refining, pharmaceuticals, etc. is resting on the shoulders of a few th

20 years. A complicated technology like present aircraft, nuclear energy, petrochemical refining, pharmaceuticals, etc. is resting on the shoulders of a few thousand 40-something-year-olds that have the book learning of the 25-year-olds coupled with the experience and judgement that makes the technology practical, and particularly the knowledge of how to go from zero to one multiplied across many many sub-problems. If something puts a particular technology out of practice for 20 years, many of those now-60-somethings, while still alive, will have forgotten much of what they knew, and not be suitable to put the hard hats back on and get in the field. And this is not accounting for the effects of whatever catastrophe put the technology out of practice to begin with.