Actually, I first read this article on petapixel and in it, it quotes the photographer as saying he spent six years and 4200 hours on it. WAY less than the total number of hours in six years. I hadn't realized that this article didn't mention the number of hours.

Umm, if you do the math that’s one shutter click every 1/3 second for six years. Yes, three pictures every second, 24 hours a day for six years.