Mary Meeker's internet report this year is one place where you can see a summary chart that suggests that not only is technology innovation happening exponentially (which a lot of people agree to) our adaptability is also growing (which I guess it has to in order for innovation growth to continue like that?):

https://www.bondcap.com/report/itr19/#view/156

(however what's most interesting about that slide is that we no longer can adapt to the rate of change/innovation in technology)

So don't think things need 20 years anymore - how quickly before Twitter, Instagram, Facebook and Google became mainstream?