How long did it take to go from basically zero difficulty to where we are now? ;)
Discussion
It's more like, how long would it take you to find 2016 blocks of the current high difficulty on pen an paper?
Then there's the first difficulty adjustment, down by maximum a factor of 4.
And again, how long would it take to find 2016 blocks of this still damn high difficulty.
Then there's the second difficulty adjustment, down by maximum a factor of 4.
There will be many adjustments before it's back down to genesis difficulty, but it would take decades, centuries?
Plenty of time to re-invent generators and semi-conductors! 😂