Also in the event of an apocalypse - the first generation could have all the awareness of technology, knowledge etc but the people who are experts may die out? Even if experts exist the transfering of what to build into "how" to build may need a lot of nurturing - Eg a chip-design expert knows it all - but may not be an expert in building a fab (not even considering the resources needed) to build this chip. But in the world where survival is paramount chip-design is just now a luxury skill and very few incentives exist to urgently preserver/pass down this knowledge? My guess is if apocalypse doesnt turn into anarchy and a rort, you need stability and peace during which technological experimentation/building to florish. SO yeah 50-100 years seems reasonable.
Personally I don't believe in an utter apocalypse like in some zombie movie. There are too many solar panels and car batteries scattered around the globe for them to be all destroyed.
Just the base transceiver stations, which commonly have backup power, would be enough to maintain some degree of civilisation.
