I know pretty much nothing about it, but I like Fermi problems.
I estimate 600K hours between 1859-1900, 1.7M hours between 1901-1950, and 2.8M hours between 1950-present, making 5.1M hours
It needs another few years until I solve it (SARCASM, I PROBABLY WON'T SOLVE IT), so roughly 5.3M hours...