There are 1000 entries in the Dewey Decimal system, from 001 Knowledge to 999 Extraterrestrial worlds (side note: my local library has a ton of UFO books in 001. So their collection starts and ends with extraterrestrials). About 90 of these categories are unused entries.
Some categories are entirely foreign language. Being monolingual, I'll have to skip those.
There's also a number of sections that are largely reference works, such as 453 -- Italian dictionaries, or 031 -- Encyclopedias in American English. Unless I find something readable in that section, I'll have to skip those as well.
My guess is that will be about 800 books total. I'm a fairly fast reader, but that's still daunting. 2 books a week, 50 weeks per year (2 weeks off for vacation) comes out to 8 years of work.
As I said before, I'm not in a huge hurry. I read lots of non-fiction anyway; this is just channeling and guiding it. I'm not planning on dropping all my other activities so I can crank out this challenge in record time.